Master volume all the way up to get those EL84's nice and hot, volume (preamp) only at about 12 o'clock -- about halfway on the chicken-head knob dial. 9-10 on the guitar volume knob for the "angry rhythm" tone, 6-8 for total cleans. I think psychic_sushi had the preamp volume at 3 o'clock the first time... before he plugged in your stompbox which sounded terrific, by the way.
The amp is also tubed for high headroom rather than "giving it up" mode (rhythm bias kasi e) -- Mullard ECC83's in the preamp and Telefunken EL84's in the output section. It has an SS rectifier and that very loud/bristly Vox (Celestion) Blue speaker.
sorry for bringing the topic back to gear. I'm quite interested how you get your "rhythm sound of doom" with an AC15, especially since it has an (1x12)alnico speaker which aren't known for their bass response right?... what do you mean by tubed for high headroom? i ask this becuase I replaced one 12au7 (Marhsall) with a 12ax7(JJ) in my frequenlty maligned Royal amp, and what do you know? it sounds fuller and much, much louder now... with a little bass added! still far from those 6L6 lows, but an improvement on how it used to sound... didn't know a preamp tube can do so much...
A preamp tube can do a lot! Pero rewind muna tayo ng konti....
With 2 EL84's, the AC15 doesn't inherently have "the rhythm sound of doom" in a gig situation like 6L6 or 6V6 fender types, or even EL34 type amps which can generate quite a bit more power and bigger cleans. 15 watts is pretty much all a pair of them normally put out, 18 watts if the circuit is particularly simple and pure. What EL84's DO have is a great deal of cut, bark and presence. So the choice of old Telefunkens which are more "hi-fi" sounding and don't "give it up" as easily as other types helps insofar as they allow the amp not to turn too slurry when it's close to maximum operating levels -- a lot of the clarity, tightness and detail is preserved in the all-important midrange. That's good for rhythm players but not necessarily for lead guys, many of whom look for that slurriness and breakup for a good lead sound. So Telefunkens are as close as one is going to get to achieving a decent "rhythm sound of doom" from a pushed 15-watter. This, by the way, also holds for other 15-watters like Blues Jrs., Pro Jrs., etc.
On to the preamps. The big deal with preamp tubes is how much gain they are able to achieve in a circuit. The volume of the amp -- both apparent and absolute -- is also affected, but much more so the former because of the gain a preamp tube (usually in V1) is able to introduce. On the amplification scale, a 12AX7 or ECC83 has an factor of 100, a 5751 = 70, a 12AT7/ECC81 = 60, a 12AY7 = 44, and a 12AU7 =20. The 12AX7 will certainly be "louder" than any of the others in both an apparent and absolute sense, but the reality is that clipping and distortion of the signal starts to happen way before you ever dime that preamp control knob to the max. If hi-gain isn't your bag, the choice of a lesser rated preamp tube allows you a much greater usable range of travel on your control knob and the ability to dial in your preferred sound more subtly. In your scenario, replacing the 12AU7 (amp factor 20) with a 12AX7 (amp factor 100) yielded the expected results -- louder, fuller, etc -- but I'll bet that at gig levels, your knob hovers around 4 or 5 instead of the 9 or 10 that it used to with the 12AU7 stuck in there, and that the knob is a lot more sensitive to minute movement in either direction. At 8 or 9 the sound is probably unusable as a good rhythm sound. And this is just on the amplification (volume) issue alone -- there's the added dimension of how different preamp tubes clip or distort -- that's an even longer story.
On the AC15, the first preamp tube swap I tried was a 12AT7 in V1. I found it a bit cold in light of the telefunkens in the output section. I ended up settling on a 12AX7 as it warmed up the overall tone despite high amplification factor. Another approach would have been to go with with warmer EL84's on the output (like new JJ's, or old RCA's or GE's) and a less-gain 5751 or 12AT7 in the V1 preamp slot. Normally, I prefer the output section really pushed because it brings out that bristling dynamic character of an amp, hence the choice of the teles in there -- everything else followed. It's really a chess game because the preamp and output section are always interactive. The trick is finding the right combination that does what you want it to do. After you've done the basic math, trust your ears at the end of the day.
I knew it. Meron palang reserve licks itong si veggie joe sa bulsa.... ayaw lang mag-sharing-sharing.
isang beses nagpunta ako sa bahay ni veggie. from the gate, i could hear someone doing a mean, jazzy rendition of Careless Whisper... then segues into Man in the Mirror... tapos one face melting Eruption... tapos lumabas si Veggie Joe with a weird grin on his face... parang smirk ala Jack Nicholson... 
The man is just waiting to pounce on us one day and blow us all away. As I've said before.... it's those quiet, mild-mannered guys you have to watch out for....
