I see.
The 443 tubes will not have the same fizz and the 448 will be a bit more flat. If you can wait until Saturday, I can run the 443 (grey) and then 448 (red) to see what I think with the MWDR. Normally when I get new tubes I usually run them in several of the amps to see how I like or dislike them. As of late I have been focused on the Reissue IIC+, JP2C and the Mark VII.
First hand experience with the STR448, I got them in grey to match what I had with the JP2C which originally came with the STR440 (grey) tubes. The JP rips with the STR440 but developed some static pops. Shortly after that I got the MWDR. That too had the STR440 (grey). MWDR is a bit brighter than say the Roadster as that was the only reference of Dual Rectifier I have. Totally different amp and that one sounded best with the STR440 red or STR440 yellow. Any how after finding the STR448 grey to be half decent (almost as good as the NOS STR415 I got at the same time) with the JP2C. I tried them in the Badlander 100W and the grey codes were awesome. Bought a second set so I could run them in the other Badlander 100W as I run that amp as a stereo pair. Well, that second set came in red. I tried them in the Badlander and it went totally lame. No drive or punch gain like I got with the STR448 grey. I tried them in the JP2C and that too was lame. No distortion or reduction of total distortion as both amps rely on power tube distoriton as well as that from the preamp. I do recall running the STR448 grey in the MWDR, it was mud fest. Too much distortion and not quite usable. Same with the Roadster. So why the STR440 grey works better than the STR448, it is more or less the difference in impedance as the tubes do have different sound qualities. Yellows in the STR445 are the highest headroom of that tube I have, too much for the MWDR. I will see how the STR443 tubes in grey compare to the STR448 in red with the MWDR. The STR448 red were a bit over the top with the Roadster. Many claim the 6L6 tubes in the dual rectifier are basically coasting at a cold bias. I beg to differ. Dual Rectifiers will have a higher plate voltage and will use the same output transformer as the Mark V90. Same bias voltage as it seems the -51V is Mesa's universal voltage for bias of 6L6 tubes. Since the tubes are different the STR443 in grey may be equivalent to the STR448 in red. That is just an observation.
I will check the STR443 vs the STR448 and see how it pans out. Will also revisit the STR445 yellow again. The STR445 are the stock tubes in the Mark VII and the Reissue, they sound great in those amps. Tried them in the Mark V90 and they did not work out, they sounded terrible.
Trying to reduce the brightness of the MWDR may not be power tube related. More so, the phase inverter tube. A Mullard 12AX7 long plate reissue will add in some bottom end and shave some of the top end a bit. Sovtek LPS will have a weaker midrange content but is the same tube visually. Those are the common tubes used in phase inverters when most start tube rolling.
Mullard CV4004 is also a dark tube, similar in design to the Tung Sol 12AX7 but different tone. The Mesa branded JJ ECC83s is also a dark tube but not quite the same as the CV4004. CV just means Civilian vs Military and the 4004 is the designation of the 12AX7 much like the 12AX7 is also a 7025.
The key tube locations to keep Mesa tubes is V3 and V4 as I stated previously. I have not done much with the MWDR with different tubes so I can see how things turn out. Usually when changing to a different power tube, I will also tune the preamp with different tubes to see if there is any improvement. I will not use any NOS tubes so they will be current production which is easier when it comes time to replace them.
Another alternative is the Tung Sol 7581A. It is Russian but had some good qualities in the Roadster as well as the Mark V90. I still have them, will give them a try but they are not Mesa branded tubes. I bought them from Canada (the tube store) but Tube Depot is in US and they have most of the same tubes.
