Best tube mono blocks under $15,000?

MC 2301's seem like the obvious choice.

They seem a little over the top anybody ever ABed them with Mc275?

Try BAT VK-150SE or BAT VK-55SE. I'm not sure of the price.

Atma-sphere MA-1's should give you everything you need as well as heat your home. As with all amps speaker match is very important.

Power rating: 150 watts/channel into 8 ohms/145 watts into 4 ohms
Power bandwidth: 2 Hz - 75 kHz within .5 dB
Frequency Response: (1 watt): 1 Hz - 200 kHz within .5 dB (-3dB @ 1 Mhz)
Phase Shift: less than 1 degree @ 20 kHz
Power supply storage: 600 Joules/chassis
Output section rise time: 600V/micro-second

Power consumption: 500 watts/chassis
Tube compliment: 14 6AS7s and 4 6SN7s per channel
Input impedance: 100k/ohms single-ended; 200k balanced
Output impedance; 1 ohm
Signal-to-noise ratio -95 dB

The impedance curve of the V2 that is shown in "Chart 5" here indicates that its impedance is around 4 ohms in the mid-bass region, slightly higher in the deep bass and upper treble regions, and rises to around 10 ohms in the mid-range.

Which means that a tube amp having relatively high output impedance/low damping factor (such as the MA-1's, which are presently listed at the Atmasphere website as having an output impedance of 2.3 ohms) will produce a mid-range response that is more emphasized, and bass and treble responses that are less emphasized, than would result with a solid state amp having low output impedance/high damping factor (such as your MC501's).

The MC2301 has a damping factor of 15, corresponding to an output impedance of about 0.5 ohms. Relative to a solid state amp such as your MC501's (damping factor 100, output impedance 0.08 ohms), the MC2301's will emphasize the various parts of the spectrum in a similar direction as the Atmasphere's, but to a lesser degree.

I suspect that a pair of MC275's operated in parallel mono mode would have an output impedance close enough to that of a solid state amp that the resulting impedance interactions would be similar.

I have no idea which kind of response would be preferable to you, or with the speaker generally, but those are some factors to consider.

Regards,
-- Al