Hi hypnos 1,
Firstly, I decided to try an all Russian all star lineup and use two 6P23N tubes for power tubes, and one for a driver. This worked quite well, and the bass became firmer and better controlled, without a feeling of hollowness. Nevertheless, real slam and attack in the bass is missing, but all-in-all a nice combination.
Then I read your impressions of mixing two different power tubes and getting a synthesis of the best of them both. Sounds interesting; let's try it. So I swapped out one 6P23N for my trusty Hoffman (nee RCA) top heater wire 6SN7.
Well, I listen through speakers and not headphones, and unfortunately I did not get any synthesis of the sound, but two distinct halves, right and left. The Hoffman side sounded full bodied and coherent with a fat round bass, and the Russki side sounded thin, akin to the difference in general between the 6N6P-IR tubes compared to the RCA 6SN7 octals when used as power tubes.
In conclusion, this did not work for me, but it taught me that power tubes could make such a difference that I could clearly hear the difference, similar to hearing the differences between different sets of driver tubes.
Now I am listening to the 6N23P as a driver with the RCA 6SN7 as power tubes. This is quite a good combination - the sound is open and forward with great detail and soundstage, except for a peculiar brightness bump in the upper midrange - we'll see if it gets better by time. The bass is very strong with great impact and slam; the mid bass is outstanding with a propensity to unlock the full bass potential of old 20's 78's (and everything else for that matter).
Funny that the other way around (6N23P power and octal driver) it is only OK, but this way with 6N23P as driver and power octals it is quite good.
About the upper midrange bump - I have four tubes, so I'll may try another one if it does not go away.