The F4 is a current source amp, meaning you need a voltage buffer like a First Watt B1, or a serious preamp to give what it needs, the yin to the yang If you want a top quality buffer on the cheap, look into a Salas Hot Rodded DCB1 on DiyAudio. Incredible price/performance.
Huh? The F4 isn't a current source amp. It's a voltage source amp. F1 and (sorta) F2 are current source amps. And you wouldn't want to use a buffer like the B1 with the F4 because the F4 is already a buffer (gain of 1). So you'd need something with a little gain in front of it.
Edited by Steve Eddy - 3/2/14 at 8:17am