Sorry about the confusion there. I was focusing only on the JP2C and mentioned the other amps as a reference. I meant the note definition and tone density of the JP2C. It has yet to be rivaled by any of my other amps. That is with the STR415 Sylvania tubes. OMG what a sound. I wish I had an original IIC+ to compare with it. From what I recall, there were a few models that ran close to 500VDC plate volage and the STR415 was the only tube that will safely work with that. I doubt the JP2C is pushing that kind of voltage. Still those STR415 once they warm up are incredible.
Understand one thing here, the benchmark tube I had for reference were the original tubes that came in the JP2C in 2016, STR440 grey. Those lasted for a few years but had some issues with static pop noises from time to time. When I got the MWDR in 2018, I swapped the tubes as they had the same tube and bias color. The tubes I had issues with in the JP2C worked find in the MWDR and are still in that amp now. JP2C is temperamental with power tubes. I eventually got the static pop with the other set of STR440. Tried the STR443 for S&G's, no static pops, but neutered the amp and basically lost interest in using it. I can see why one would think on selling the amp. However, it is unclear if that is the issue with the amp you have. The suspects would be V3 and V4 since that is the heart and sole of the JP2C for the lead drive voicing. Basic signal path is V1A-> tone stack -> V1B -> V3B -> V4A -> V2A -> fx loop -> V2B -> GEQ -> PI -> power tubes.
If that issue cannot be resolved with preamp tube swaps, it could be an internal issue. Not an easy task to be poking around a stuffed board. Before you do anything exploratory, I recommend against it unless you are well versed in proper safety and know what it is you are looking at and working with. First thing to do, email Mesa customer service and see how they respond to your inquiry. Just state the facts you have little or no gain character on CH2 or CH3 but an abundance of sound on the clean. They may have something to offer or suggest.
Just to keep you from removing the chassis, here is what it looks like inside. No need to open up the amp you have as it should look much the same. It is easy to get out of the head shell, not so easy to get back in.
Considering the layout, it is not all that bad when compared to the Mark V90 or the Mark VII. Even un-plugged, there could be lethal voltages stored in the capacitors. Be safe, leave it. It will not kill you to remove the chassis, just keep hands out of the circuits. Still no point in doing that unless you have the limited edition JP2C as that face plate does not come off. My hands are too large to reach around the fan and stuff in the way to get to the preamp tubes. Not something you need to change on a regular basis as they should last a long time compared to the lifespan of the power tubes.
The STR445 (green bias) are very close with a slight tone difference. Those sort of have a 6CA7 character sound but are not an EL34 variant, they are 6L6GC.
The STR448 (gray bias) are close as well but have more air in the sound. Those rock too. I was on the fence which tube I liked better. Since I have the rare STR415 tubes, may as well enjoy them while I can.
STR445 and the STR448 are priced about the same. Sweetwater only carries the STR448 and the STR443, if you order them, make a request for grey bias color or even blue if they have them. Sometimes when you ask for grey you get green. GRY vs GRN can be confusing for those picking from boxes. Musicians Friend (aka Guitar Center) zZounds also has them. STR445 is about $10 less per pair than the STR448.
Now you gave me the opportunity to open up Pandora's box:
to another long post.
If I set that up to happen, my bad. Was not the intent. Sorry. I will try to keep it short.
Typically when I get my hands on new tubes or something different, I will try them out in everything I have that will work with them. Mostly since the STR440 tubes are no longer available. Why pay a premium for a so-so power tube? For the Roadster, I would if nothing else works. I sort of have a better perspective on what works for me in each amp. (what is listed in my signature).
Mark VII:
If we were talking about the Mark VII, I would have to say stick with the STR445 in a yellow bias color as I found the green bias colors to be too much for that amp, it is noticed at gig level as the amp will loose some character and get swamped. The first mark VII came with the green bias color STR445 tube. I repleced those with the yellow bias color. At one point I was running a pair of the STR448 in yellow bias or red bias color range in the outer sockets to improve the tone. The mixing of power tubes in a Simul-Class amp works great. What is in the extended-class A sockets (STR445) sets the overall tone of the amp, the supporting tubes for 90W adds to enhance the sound characteristics but not that dramatic. On the flip side, the Class A/B amp, when mixing different tubes having the same bias color or different, the tube with the lowest impedance will set the overall tone. That trick does not seem to work well. If you do try that, make sure the inner pair are matched and the outer pair are matched. That way when you switch to 60W you do not have a miss-match set.
Dual Rectifier: (depends on the model)
Each amp will have its own character to it. Say for instance, the Dual Rectifier Roadster, sounds the best with the STR440 reds IMHO. MWDR that would go to the STR440 grey. I personally did not like either of those amps with the STR445 tube. I have not tried the yellow bias color version in the MWDR yet. Those were too much for the Roadster. It may work well. Won't know until I try it. Thing with the Dual Rec, it is all about preamp. Stuffing in tubes with early onset of distortion (green, gray, blue) throws it off and makes it sound muddy. More true for the Roadster than the MWDR.
The Mark V90:
The best tube for that amp is the STR454 (SED =C= 6L6GC). Those tubes had a 3D effect to them. I did not get that effect with the other amps though. STR441 tubes actually sounded really good in that amp too. A bit off in the Mark VII. The STR415 made the amp sound boxy. Same with the STR448 tube.
I will not jump into EL34 mode as the JP2C does not support that tube type. All you get are the flavors of 6L6 tubes offered by Mesa (if you like your warranty). Sure, one can use non-Mesa branded power tube, thing is to get them in the proper plate current range. The bias colors are not a specific plate current value, they are ranges and may overlap to a slight extent. Some of the selection criteria may be based on emissions from the cathode as well as plate current. That sorting process is over my head so best leave it at that. Just an interesting point to make, Mesa was getting in bulk supply the STR440 tube. They would test, sort, match, brand and label them. Discard tubes that did not fit in their specific range or had other quality issues. Ruby Tubes offered the same Shuguang tube, 6L6GC-MSTR ? been a while so I do not remember the actual part number. I still have them and I can look at the box they came in. They looked the same, sorted and tested differently by the vendor and had a different characteristic tone.
Sometimes you may even hear the difference between the preamp tubes. Those get tested and sorted, branded or discarded. Tubes from Eurotubes or other resources that sell the JJ ECC83s may have a different sound or gain characteristic than the Mesa branded tube of the same thing. It is all due to how the tubes are tested and rejected. That is more of a trade secret. Similar to the Groove tubes thing back when they were in business before Fender bought them out. Mesa or Groove tubes do not manufacture vacuum tubes. Neither does TAD, Ruby, Eurotubes (unless they are a direct link to the resource) New Sensor and the like. They filter out the bad and sell the good (I can only assume). That service adds cost but keeps the junk out of our amps in most cases.
I do not know everything. This sort of turned into a tube discussion, my bad. Also, everyone has their own opinion on what preamp or power tubes will sound best for their individual needs. What may sound epic to me may sound terrible to you. Depends on the guitars you run, are they extended range or standard 6? The JP2C can handle either.