Little bit more about my impressions on the Torpedo (EDITED!):
I tried all the tube sets once again and what surprised me was that the variation in FR is not as significant as I expected before getting the amp. TomB sent me 3 different matched 6J6 sets (with different subnames on it), one pair of 6J6W, 5964 and 5844 = 6 matched pairs in total. I am not able to precisely speak about each subtle difference but in general, this is my personal opinion so far with HD800 and StageDAC:
1) 6J6 (Top UFO) - cannot say how it is possible but this set has significantly better soundstage width... I mean SIGNIFICANTLY. The rest of tubes are rather similar in this regard but this set beats them all just because of this parameter. Strange, I didn't play them more than the rest... I thought it's a placebo but it's not, no problem to do a blind test here and recognize the difference o_O
2) 6J6W - looks beautiful when plugged because only those are completely transparent from the top and therefore you see that beautiful visual effect of tubes to the full extend. As for the sound, so far similar to the rest of 6J6 series.
3) 5964, 5844 - Not sounding completely the same but I prefer 6J6 tubes over them so far, not by much at all though... The differences are very small and it could happen that for some specific reason (sound-wise) one of them will become my favourite in future.
Now a little bit more on the sound, in comparison to O2 as a VERY GOOD solid state amplifier (using the same cable and 1/4 headphone out on both amps = no adapters). I still think those two are on par but provide a SIGNIFICANTLY DIFFERENT experience (I mean SIGNIFICANTLY IN TERMS OF AMPLIFIERS...):
O2 is clear, airy, transparent, "monolithic" sound-wise, neutral... Just easy to assess, nothing to stay hidden (I mean in terms of its sonic characteristics), nothing complicated.
Torpedo is much more complicated... It's not as clear, airy or transparent (yes, it's not a completely transparent amp). However, there really is zero harshness, imaging is on par with O2 and with 6J6 tubes (see above), Torpedo beats my O2 in soundstage significantly (O2 still a bit wider but Torpedo much more natural and near-perfect, at least for music I like, IMHO). Frequency response leaning towards the warmer side (not dramatically though)... Highs are very nice (natural guitars and cymbals in comparison to rather lifeless and harsh interpretation given by O2, most probably due to impedance match issues), voices as well full and beautiful, bass is there but lacks tightness and ultimate impact so far, regardless the tubes I use. O2 bass is analytical but present, with relative ease...
Both are in the same league IMHO but I am surprised that O2 really suffers from impedance mismatch with HD800... The highs (including female vocals) lack smoothness and detail - yes, detail! It seems TomB was right here, it seems near-zero impedance really isn't ideal for all. Torpedo, on the other hand, lacks complete transparency O2 provides. The difference is not big but for those of us who consider transparency very important (hi, David!), this could be an issue (to solve with the rest of a chain). Especially with warmer headphones.
So far, I struggle to choose which of those two amps to prefer... They are both really good in what they provide. I guess it would be good to get some magical set of tubes which could tighten up the bass (lower subbass frequencies a little to clear up the sound) and maybe brighten the overall signature a bit ( = to increase a perception of transparency = less subbass could do the job as suggested previously). Maybe if I burn-in all the sets of tubes, one of those would do the job... We will see.
EDIT: After tweaking my StageDAC's setting, I found out that:
A) mid-position of Tonal balance switch gives the amp the bass impact it needed (it puts emphasis on frequencies up to 2khz to balance out the crossfeed effect), without loosing the separation completely (bass moves more up to the center where singer's position is which I find more natural than rather analytical approach with HD800 "stock" bass, positioned significantly below singer and guitars). Listening is now more near to the way how speakers do sound, significantly... And the bass is more on the scene, in terms of both position and impact/subjective feeling of tightness. Image-wise it's a bit worse but nothing I cannot live with.
B) Low-positions of Pulse response and Over sampling gives the sound the last touch of brightness it needed to feel reasonably transparent ( = low Pulse and Low Oversampling means classic DAC approach = completely flat line from bottom to the absolute top). Those few added db of high treble frequencies helped to complete the synergy. God bless the StageDAC!
I am now ready to spend few days with the setup to ensure that the synergy now really is satisfactory to near-full extent, at least. The amp still could be a touch more transparent in order to be "perfect" itself (bass is alright in the end for me, just not as punchy without my StageDAC), let's see what burning in the tubes brings!
Disclaimer: Please, take it rather as a temporary opinion :-) But it could help or at least interest someone enough to give an advise or another opinion. There aren't many posts about how Torpedo actually sound... Well, right now, one more added ,-)
Disclaimer 2: O2 haters, skip all my opinions and please do not underestimate this tube amplifier... It could very well outperform O2 for you in every aspect. I personally have high respect for O2 and now for the Torpedo as well.
Edited by RustA - 3/31/13 at 2:32am