"Holographic" presentation

Mono amps may not make a night-and-day difference.
Much of the voodoo that we-do (vibration control, room treatment, power conditioning) doesn't fix everything.
Taken together, however, results in substantial improvement.

"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.

Krell KRS-1 Mono Block preamp, where the control section and power supply are in their own chassis as well, resulting in 4 separate boxes.

A neighbor has one.

As far as whether monoblock configuration is more relavant in an amp, whi knows. Some might say that the very low level signal encountered in a preamp are more fragile, and when you add in a phono stage, well, the amount of gain in a preamp is going to be a lot more than in an amp.

Which get's back to what we should be satisfied with- if it sounds good, it is good.

This is what recording engineers do: Record things to their or the producer's or the artist's taste (mic selection, eq, possible compression, ambient information either natural or from devices), and pan each instrument to wherever they think it needs to go...little left, more middle, little right, lots of left, lots of right...that's it. Soundstage accomplished. Any decent stereo rig can reproduce this and if it's isolated (headphones or speakers stuck to your head) it can sound less "live" as in the actual world there is air and natural blending between speakers, as long as your ass sits between 'em. Tone, treble cues, bass nodes, yo mama, all differ.

I think there may be some confusion with terminology. The often-touted
advantages in soundstage presentation of mono-bloc amps have to do with
improvements in stereo separation and specificity; sometimes resulting in a
better defined soundstage and better defined images within that
soundstage, but not necessarily holographic. "Holographic" has
usually meant a presentation in which individual images are, not only well
defined in space with little image wander, but also dimensional with the
sense that there is air behind the images and not just to the sides and front;
IOW, 3D. As far as amplification goes, in my experience that trick is best
accomplished by good tube equipment; mono-bloc or not.