Yes, Dsavitsk will have to weigh in about gain.
Punnisher is correct when it comes to the standard gain on the Torpedo and the position of the volume pot. Almost every dynamic works out to about 12 o'clock. That's one of the beauties of the amp - doesn't matter whether Grado at 32 ohm or Sennheiser HD 580/600/650 at 300 ohms - flip the Z switch and everything is adjusted at 12 o'clock for reasonable listening volume. The only exception to this that I've found with dynamics is the K701 family - more like 1-2 o'clock for the same volume.
You might try the 6J6 tubes - especially the 6J6W's that I supplied. Those are among the nicest of the 6J6 family. The amp definitely sounds better with the E90CC's, but nevertheless, the amp is optimized for the 6J6's. It's true that the output transformers are going to do all of the converting to voltage and current at the output, so tube selection is not going to affect power that much. However, the 6J6's may have a bit more punch with the amp (but not the exquisite detail/low distortion of the E90CC's). Depending on the headphone, that may be a better fit, I don't know.
Max power was estimated by Dsavitsk at about 750mW, I believe. I would be surprised if some OTL amps exceed that with low-impedance loads, but I suppose if the tubes were very powerful in their own right, it might.