I hope I am in the right forum with this post.  If not, perhaps a moderator can move it appropriately.

Several years ago at a small audio fest I had the opportunity to hear mono recordings played through some excellent tube gear and a single full range speaker.  I was enthralled.  Mono with a single point source is very engaging if done right, at least to my ears.

In my twilight years, for my musical tastes and listening level in a smallish apartment with neighbors, I am seriously considering going with such a setup. I don't have a turntable nor vinyl collection so it would be digital files into a decent DAC, a mono tube amp and a single full range speaker.

There are quite a few mono digital files available on places like Tidal, etc but for many things that I want to listen to I would have to sum 2 stereo channels to mono.  What would that sound like?  Is this a completely hairbrained idea?

Thanks for the encouragement!  One aspect of this that I wonder about is whether summing the two stereo channels does a reasonable job of recreating a mono signal, or does that bring with it a bunch of sonic issues?

It's interesting that you used the word "holographic".  I read a comment somewhere (I don't remember where) that:
- stereo creates the illusion of "you are there"
- mono creates the illusion of "they are here"

I'm not going to say your idea is completely hairbrained, but if it were me in a smallish apartment with neighbors I'd rather listen to stereo recordings on headphones than summed to mono. My very limited experience, using an Audio Research preamp that would do the sum to mono thing, is there is a difference between playing stereo recordings as mono and true mono recordings, esp. those recorded with a single mic. But that is just me, so FWIW, YMMV and all that.

@JBzen  I did some more research and you were right.  There is more to it than just summing the stereo output.  Doing that could result in problems (and potentially damage) with the output opamps of the DAC.

There is a simple passive resistor solution discussed at several places on the internet that will prevent the opamps from loading back on each other.  I found one online source that sells cables with the resistors built in.  Will look into it some more.

With a solution like that in place, it seems technically feasible and doable, subject of course to the sound quality caution mentioned by @CAJames.

I have summed a stereo signal with resistors for sub use. Bottom bass are generally non directional sound waves. I would think that as sound becomes more directional above 80hz problems would develop when two different amplitude waves with the same frequency meet and sum. Adding time deviations from stereo mixing will only compound the problems.

John