These notes are mostly for my own use/curiosity. I'm limiting these notes to class AB and A amps. Once you get into switching amps, things change quite a bit.
The Nelson Pass designed GFA-555 is known for a good but bright presentation. I will … wait for it … pass on that. Other less costly GFA amps are widely available on the used market. The c. 1993 GFA-535 was reviewed in Stereophile.
H/K embraced Otala, et al. early on and have a history of designing “wideband” amplifiers, going back to tube designs. This maybe tips them toward having good top ends. My experience with a receiver has been favorable. But the company seems to have let their quality slip starting in the 90s or late 80s (?), so it's unknown if these amps are credible contenders.
NAD is persistently dogged by reports of reliability issues. Personal experience with a 7020 receiver (ancient!) corroborates this somewhat. It also corroborates that the NAD sound favors a “warm” or thick bottom. This might be expected if the design approach uses large amounts of “dynamic headroom” (as NAD AB amp designs have tended to do): With small power supply caps, supply modulation in the LF will increase, and this will increase the LF THD.
These are claimed not to have output protection. Otherwise promising candidates.
Not as popular or having the same buzz as lots of other manufacturers. But the brand intrigues me. They pay at least lip service to wide bandwidth some some/most/all power amps, so are worth a deeper dive.
John Curl's involvement with the company lends them some cred. Whether the products not design by JC are to the same standard is not currently known.