"Holographic" presentation

TWo mono amps, or a stereo amp that consists of two highly isolated mono amps in a single box even, may well have better channel separation and less crosstalk, which could work in favor of a more "holographic" presentation, but whether or not this would be heard in comparison to something else or not would still depend.

Assuming quality gear designed well in all cases, using two mono amps is probably a good insurance policy that overall performance of each is fairly independently optimized which should always work in ones favor.

In a nutshell, more can be done better with a single self contained monoblock, with dedicated power supply, etc., than can be done for similar cost otherwise.

"Here's a question- what good are dual mono amps when you are using a stereo preamp?"

Two different things that work together. AN improvement anywhere is an improvement, and vice versa.

I suspect the reason monoblocks are fairly common yet stereo pre-amps are housed in the same box is that there is more potential for 1 circuit to interfere with another in a power amp due to its nature (ie high power, voltage, more current, high capacity power transformers, etc.).

Monobloc pre-amp is an interesting concept. Not sure I have ever seen that. Probably because benefits are not significant. Still, audiophiles like everything to be pure, so you'd think not having a good reason would not stop someone from selling the concept.

I have a pair of TAD Hibachi monoblocks. Each has continuously variable input sensitivity controls. Essentially, that is like having a built in pre-amp with volume control, so I suppose that's it. Nice amps! No multiple line level inputs and switch though.