"Holographic" presentation

I think that speaker placement and room treatment are more important factors determining three dimensionality than whether the amp is of stereo or mono design.

I was comparing two different speakers - and there was a noticeable improvement in soundstage depth and width in one vs the other.

A speaker designer I know says there's a simple explanation for that - our perception of soundstage is determined by the slight differences we hear from the left and right speakers.

The speaker with a flat soundstage had a less rigid cabinet. As such, there is some vibration from the cabinet that is smearing the soundstage. The vibrations created tend to be "mono" like from both speakers and that masks the small differences that were in the original material.

As Mapman has stated, the use of mono blocks could reduce the amount of crosstalk (or mono information common to both channels) and I can see how a similar reasoning could explain why the soundstage could be improved with a mono block design.

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

My experience has been that as long as there is very good stereo separation, and the media contains the spatial cues necessary, you can get the holographic illusion. And it is an illusion because holographic suggests three dimensions, while our stereos are by definition 2 channel sources.

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.