For what it is worth, the bandwidth of the Fet Valve R series amplifiers is 3 dB down at 4 Hz and 350,000 Hz. This is the natural bandwidth of the circuit design, no spec improving tweaks like feedforward compensation. We can use all of the bandwidth of the mos-fet output devices because all internal circuits have a Q of 0.5 or less, no underdamped resonances under any conditions. Thus we don't have to tie down the devices to keep the amp from turning into an oscillator. This also makes the amp very fault tolerant too.
Just for what it is worth. However, when you hear it, you will understand.
Regards,
Frank Van Alstine