Using high powered amp at low volumes power

Can be a smearing effect coming from the power distribution. Try swapping power cables if handy.

If you are running a high power amp at low power levels you will have a problem where the distortion and noise floor of the amplifier is actually higher than you may have expected. Nearly all high power push-pull amps, whether tube or solid state, exhibit this problem.

This can cause the resulting sound to seem somewhat indistinct, unresolved. The solution is to use an amp with less output power or a set of speakers that are less efficient. Since you are so close to the speakers its likely the former rather than the latter.

For low volume levels I would stay away from passive preamplification. Buffered active unity gain as implemented in McCormack preamps is the right choice. I use the lowest gain setting on my preamp which is adjustable.

Quiet volume listening on my setup is little to no compromise:

Mytek digital preamp --> Sunfire 300-II --> Aerial 10T

For some of the recordings I'd benefit from changing the gain settings on my preamp, but would rather not sneak under the chassis everytime I need to do that. User friendly preamp with external gain settings is encouraged after all.

Does anyone know if a (higher end) class D amp such as Audio Research-type would run with less noise than solid state at low volumes if the 'high wattage' was exactly the same? Say 350 watts class D vs. 350 watts class A?

Per design class D amps will have noise in lower volumes.
It dictates that SPEAKERS should be used with low sensitivity.