I had been mulling over Bob's comments about grounding the sources on the MOSFETS. I was trying to figure out how to do what he was saying, to make the connection between the MOSFETs float. I think you would need a complementary device to do that. Those are hard to come by nowadays. And negative voltages, which would be annoying.
I had also been wondering why people put transistors at the emitter or source of a differential amplifier, as a current source. I was staring at one of those, and I realized that would accomplish the same thing.
I've added an NPN transistor at the source and biased it properly, and it does work. It reduced the amplification a smidge (1dB) and it did increase the 2nd order harmonics some (10+db) but it should make the differential part float, as I understand it.
Thanks,
-corey