This is a pretty broad question that covers a lot of range and depends a lot upon desired tone and response in conjunction with what cab you are using, what guitar and pickups you are using and even your playing technique. However, I recently retubed a 3 channel dual rectifier with some pretty satisfactory results. Actually, the result was pretty amazing. Here is what we did....
First, we addressed the V1 slot mostly focusing on channel 1 in the clean setting. Mesa uses a cascading type design which doesn't allow to change just a single tube to address just a single channel which can make it a little harder to nail just the right combination and order of tubes for all three channels. What we ended up going with in the V1 slot was a blackplate vintage RCA 12ax7. This gave the Mesa a little more of smooth top end that seemed a little "pointy" with the stock tube. The RCA gave it a lush smooth low end, with a very warm low mid, but seemed a little less overbearing than the stock tube. The highs were still very present but had a smoother top end that did not hop out as much on higher notes. It really gave the clean channel a bit more of a vintage sound that tracked well at higher volumes. It also allowed but clarity on individal notes when there were still others ringing out as well as made the amp much more responsive to what was actually being played. It kind of gave it that slurred vintage sound that was still more clear than the way the stock tube smeared chords and rythmic changes. Harmonics were also much clearer and "chimy".
Next we addressed the V2 slot and focused on channel 2 in the vintage position. In this slot I beleive we ended up with a vintage Mullard 12AT7. The mullard really helped to refocus the low mids and gave them a lot of clarity well still maintaining an even and sweet high frequency response. Adding gain now resulted in a much punchier sound that once again did not smear the sound as more notes were added. We were really after that vintage warm Marshall and orange sound here. With EQ flat it had a very present but still punchy and very warm classic distortion sound with nice forward mids. It also helped to speed up the responsiveness of the amp so that notes on top of a sstained chord really hopped out a little without being overbearing or changing the tone of the sustained chord. It also helped the overring or "thump" created in the speakers when palm muting, chugging, or muting a chord. The amp seemed to get on notes quicker and get off notes cleaner. At this point we also noticed that the noise of the gain was also decreased and the channel two controls were much smoother when moving through different volumes. Basically, the volume hop between different output levels all but went away giving a wider and more usable volume range to work with without having to address it at the master volume.
For channel 3 we were after a more modern sound to compliment the channel 2 vintage sound we had established. In the V3 slot we ended up addressing it in the modern mode. As you probably know, the modern switch gives the Mesa's a kind of mid scooped modern sound with a little more fuzzy gain sound. For the V3 slot we ended up with a vintage Sylvania 12ax7. The Sylvania really tightened up the low end a bit by attenuating it a little which helped to give the lows a little more punch and clarity when using more gain on it. It also gave the highs a more aggressive top end than the mullard in the V2 slot. Individual notes slice a little better, but without the extra fizz that adding so much gain was creating with the stock tube. Its not really the channel I would reccomend for classic rhythm lines, but leads really cut through no matter what else is going on. If you are playing metal, this could be a great channel for rhythm though since the lows in detuned chords still held their clarity even when highly saturated. Harmonics also had excellent response and lots of sustain. The player of this amp really noticed how much easier it was to play his guitar in any of the channels because the amp responded so much better to all changes in playing style. All three channels exhibited a better response to changes in pickups and playing intensity.
The last thing we addressed was the phase inverter. When starting this test, we weren't sure how much of a difference the phase inverter would make. In some amps the tonal characteristics and responsiveness is not changed much by different brands of tubes, but in this specific amp it was pretty drastic. Things were sounding good at this point with the stock tube, and several of the other tubes we tried didn't make as big an impact as changing the tubes in the first slots, but once we put a Telefunken 12AT7 in the phase inverter slot, we immediately knew we had the right tube for this combination. Things were already sounding much better, but adding the Telefunken to V5 really took the amp to the next level. It seemed like the Telefunken almost added an extra octave to both the top and bottom of the amps spectrum. The amp opened up a ton and clarity in all three channels was increased once again. It became especially noticable when adding more and more gain. Saturation increased, but without killing the ends of the frequency spectrum. In the end we found that at lower volumes than before we were to maintain the same energy and punch which once again added more versatility to the amp. Noise in the amp also went down quite a bit by getting the Telefunken in line. The amp was also more stable in its tone when varying the output level which allowed individual channel settings to change less when playing at different levels.
The net result of all this was that this specific player decided to keep his amp rather than selling it and buying another amp. He was thinking about possibly buying two amps to acheive what he was after, but now decided that he can't sell his Mesa. He always liked it, but it never seemed to get quite where he wanted. Different story now. It is important to note that all these tests were done with the Rectifier tubes engaged, and the power switch in the spongy position. All initial tests were also done with EQ on each channel flat, and gain and channel volume in the 12 o'clock position, only varying the master output as necessary. Test were done on both a Marshall 1960A 4x12 cabinet and also with a pair of Mesa 1x12 closed back cabs. I don't remember what guitar was used right now, but I think it was a PRS, but it was checked with an American Strat and a Gibson Les Paul Mahogany Studio and Gibson SG standard as well.
This of course was custom set up for this player and what he plays, but we did try and address it with several different styles in each channel as well to assess its flexibility for other things. There are other options I can think of right off the bat. The RCA we used in V1 gave the Mesa a sort of hybrid Mesa/Fender sound. From testing the amp, both Mullard and Amperex Holland Bugle Boys also had a very lush sound with some vintage characteristics, and using a german made Amperex tube gave the amp a little more of a modern edge, but it just didn't suit this specific player and his style in this specific setup. The Bugle boy also worked well in V2, and the German Amperex sounded pretty good in V3. Also, using a GE 12AT7 and 12 AX7 in the V5 slot sounded good, as did an older Tungsol 12AT7, but the Telefunken was where the magic was in this setup.
If you are interested in any vintage tubes for your Mesa, feel free to send me an email or PM here. I have some listed in the For Sale section here, but there are a ton of tubes I have right now that are not listed in that posting including vintage Rectifier tubes, 6L6's and some EL34's. Hopefully some of this was helpful
