So, out of curiosity I looked up the ECL82 (6BM8) that spritzer suggested. It turns out the amplification factor when the pentode is triode strapped is about 6.8, which is nearly 10 dB lower than the 6SN7GTAB, leaving about 4 dB of feedback. If we assume the grid-to-plate capacitance is about 4-5 pf, this puts the open loop frequency response at -3dB between 22 and 27 kHz, which is where the feedback margin runs out, so the closed loop frequency response is about the same. By contrast with the 6SN7GTA/B the closed loop frequency response is -3dB at around 53 kHz. Voltage rating and power dissipation are not the only factors that need to be considered in selecting an output tube.