Most NEUTRAL class A mono's

Sorry, I meant to add that the Sanders are not class A but seem to be in that league without any of the heat problems. I was looking for a high power class A single ended amp but the cost can be over the top and they use hugh amount of power and run hot etc. I love class A but so far I am very impressed with the sound of the Sanders. They idle at 7w but can peak at over 2000w

Surely Atma-Sphere MA 2s must rank?

Gryphon Mirage and Colosseum amps. The Antileon Signature Mono's are also quite good, in a different league vs the stereo.

I would have reservations about a $20K active low pass (LPX) myself, but that alone doesn't limit you to the Tidal Impact amps. Pass, Krell, FM Acoustics all make top quality active crossovers that could be substituted as well and accommodate different brands of amps, which really opens up the options. Maybe SS for bass and tubes above? BTW, adding an active high pass to the already existing passive high pass can work well if the former is set to a slightly higher frequency. I've done it on my Genesis along with the factory active low pass.

Not Class A but one of the most neutral amps for the money are Spectral. And they are well priced new. The preamp is also incredibly neutral as well.