agreed.
Also, the F1 and F1J have a high output impedance (and F2 and F2J).... so, it doesn't make sense to make an exception for them Tyll. My point is that It's the other ones that have low output impedance, aren't gm amps and sound awesome in the context above.
Also, here are some quotes to consider from the F1 owner's manual:
This is a very unusual amplifier that will not sound good with about 98% of
the loudspeakers on the market. It requires careful attention to
loudspeaker loading to get the best performance. The accompanying white
paper “Current Source Amplifiers and Full-Range High-Efficiency Drivers”
is required reading. This is a tinkerer’s amplifier, and experimental tool. If
you just want to be a consumer, then buy something else, otherwise be
prepared to patiently experiment with your system to get the best
performance.
It does not often work well with ordinary passive crossover networks – the
components and their locations tend to be reversed. It won’t break, but
probably won’t sound good