Most NEUTRAL class A mono's

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.

Peteferg, I had the Sanders Sound Megtech amp for non-esls. It is extraordinarily powerful and sounds great and definitely a best buy at $5000, but i don't think it is in the league of the Tidals and Sovereigns, although I need to say I am going with others' opinions on both of these, as I have not heard either.