Hi there! This is Chris Hoppe of
Hoppe's Brain.
The GFP-750' passive mode is a fickle thing. The volume potentiometer has an unusually low impedance of 2Kohms, and this is difficult to drive for many source components. On the other hand, if your source component is happy driving 2K, then you get a nice and relatively low output impedance, for a passive preamp.
Generally speaking, you want at least a 10:1 ratio between output impedance of your preamp, and the input of your amp. Fet and tube amps tend to have high input impedances, which makes them good candidates for passive preamps.
The GFP-750 is a great design. Literally just a long-tail-pair made from mosfets, biased into their most linear region.