Appleridge,
As a long time fellow owner of a lat transitional B, I'll offer what observations I can. WARNING Loooong post: thought that some of the technical info might help you get to where you want to go.
I'm sure Mike sent along the instruction sheet for the mod, so you know that the dot on the dial is the point of unity gain--anything above that is only to compensate for effects with low output.
One of the beauties of this mod is the ability to use post effect knob is a master-master volume to bring the amp down to "bedroom" levels while opening up the rhythm, lead and master volume controls so the amp "breathes."
The mode switching and masters have circuit differences from your Mark III (or a Mark II C). Besides the switching relay's lack of distortion/noise from the C/MkIII vactrol switching, the way the lead channel switches in/out differs.
I find this to be a plus (no pun intended), as it lets you setup both modes so they work better together then they do on the original B circuit, It allows it to be a better two-mode switchable--I can find settings that allow me to actually switch to lead and punch out some rather than having to choose setting for just one mode or the other. Then you just set the master-master (post FX volume) to the level you like between 0 and about 7.
As was mentioned, the presence on the B is different than any of the various ones used on the Mk III run, but it is not changed by the loop mod, so use it accordingly.
The mod is more than just a "loop" modification, yes, Mike flips the effects driver from low to high impedance, which is where the "extra" gain stage comes from as well as its B+ moniker, but he also changed the topology of the amp by moving the reverb from the classic Fender implementation to the effects loop section. This makes reverb more useful in both modes and puts it where all time-based effects belong--at the back end, as close to the "room" as you can get without being on the output side of the speaker.
As is well known, these changes are the progenitor of what would turn the C into the C+. So both the loop and reverb should behave similarly to what you are familiar with in the MkIII. Of course, with the proper 12AT7 in that socket, its a better reverb than the C+ or MkIII IMHO. Keep a 12AT7 in that slot, the extra gain of a 12AX is not needed there and the current of the 12AT is not a plus in the V2 loop circuit.
The extra gain stage gives it a more organic lead that is less smooth-hydraulic than the later Marks since the lead on the B is still a single triode rather than the later two triode cascade, so keep that in mind when tube rolling. Also remember that the lead triode shares the same glass with the reverb return--what you select for one may effect the response of the other.
I "suffered" though with a stock 9C B for years before I had Mike retrofit the mod, as I was afraid that I would lose the one trick I used if tor--I have since spent the time kicking myself for not having it done originally--but the amp is now far more flexible and usable. even with the effects driver adding gain the clean is really close to my Mk I, which can be indistinguishable from a Blackface Twin Reverb. The point being, for me nothing was lost.
Another mod that Mike slipped in was to do a wee bit of the later Mark "Bass Shift" change. He changed the slope of the cathode filtering by tagging a low value resistor to the bypass cap. Enough to filter out some of the low end crap without appreciably changing the EQ of the amp. Nowhere near the amount done on the C+. He may have done this on yours; He would have probably told you, but you can be sure by looking at V1 and spotting the flying components on the board. I have since put a switched pot in there to access both characteristics like a C does, but it usually stays set with Mike's handiwork--he does not waste his time on un-impactful changes.
Likewise, you are probably aware that the "Gain Boost" pull switch is not really the original gain boost circuit that lifted the drain of the tone stack via a foot switch or pull knob. It is a switch button that changes the filter shelf at the back end of the amp, later relabeled as such on the C+ face plate. Pulling that puts it back to old-school, while leaving it pushed in attenuates the low frequencies much the same as the bass shift does at the front end. If you are playing high gain, this attenuation may be useful, but if you are doing low/no gain, pull them out and see if you get some more low E growl and then control any flatulence with the bass control. These shift controls were implemented to compensate for the crop of poor offshore tubes that were on the horizon as NOS production/availability was drying up back in the day--they could not hold up to high gain and low crud (and because Doug's ear and guitar wanted less bass). Newer Chinese and Russian tubes are better than those early crops, so use the switch as your tubes and tastes allow.
I also had Mike change to smaller cathode bypass cap values throughout to keep the low end garbage out. I like the quicker response of the non-GEQ amp and don't need the back-end EQ to put the bass back in, since it can be controlled as needed earlier in the circuit. Don't know about your configuration or preferences, but this is probably an unnecessary change with little noticeable effect in most circumstances. So my experience may not map precisely to your amp.
Cant offer much more on tubes, that is a personal ear thing. You are not compressing ahead of the tone stack's phase shift, so I focus on getting as much headroom on the valve as possible--only get one chance at a first stage, after that all controls are subtracting frequencies. your Lead valve is where there is more play room, and you may need to sacrifice the reverb return behavior to get there. You can drive more signal with a higher gain version 12AX7 at the front of the loop if you are not plugging into it or can set the input level of the first effect in the chain. You should be aware when you are rolling tubes that the loop send and recovery triodes have been swapped. Mike retasks them to avoid bad amp behavior.
I use NOS glass and I don't generally drive the gain above blues and classic rock. That said the glass will support the occasional foray into overdrive/overboard when desired. I too run 415s and have never been bowled over by modern Mesa (offshore) big glass. I have run RCA black-plates and GE with success, but Boogie and 415s are a happy combination. If I was a working musician relying on this amp, I would have to make some hard decisions about what glass to use/wear out/replace on a regular basis.
If you have Mk III experience then you already know that the front end EQ is not set and forget like an old Blackface--just make incremental changes, one at a time, until you get to know the amp. Start at low volumes, so you can hear the changes as you rotate through. Do this with the post FX volume down (somewhere 2-4) and the original masters up from where you would have them without the loop mod. Once you have the clean setup with the desired amount of hair, set the lead drive and master for the additional hair and/or volume you want. When you get it dialed into something you like, increase the FX volume to whatever paint-stripping level you want. Obviously, with one tone stack, there may still be compromises to make.
Speakers are kind of a religious thing. I am an EV person for Mark IIs and IIIs. As Mike says "they are an honest speaker'" I find the C-90-n too mid forward and nasally and dull. In general my mission for the amp does not fit the Celestion offerings. I like the flat EQ and general openness of the EV in the B; I am free to EQ the amp to best advantage rather than fighting with it to compensate for something missing or too much of another on this side of the cone. Optimally, the combo on top of an EV Thiele with a speaker of choice on top in the combo cab... if you can tolerate the SPL and weight. I tolerate neither these days, so I leave the EV in the cab. But the tiny cab acoustics are a handicap no matter what version/mods are in the amp tray.
As GJgo said above, YMMV depending on your ears. Be patient a loop modded B is a thing of beauty when mastered. If I have to sell mine before it belongs to my heirs, it will be a tough day! Of course, I say that about all my remaining Marks.
Happy tone hunting.
