Using a mono speaker instead of stereo?

I think what the OP is hearing is the result of reduced distinction comparing mono to stereo. If you proceed with the project, I suggest you pay careful attention to the precise matching of all components.

DISTINCTION

It’s the word I choose when describing the advantage of playing a Mono LP with a true Mono Cartridge. Even more distinct when playing mono with only one speaker. Not imaging, but distinction of different instruments. Actually the prevention/removal of any inadvertent imaging. Additionally I have learned not to sit in the dedicated listening position, to keep my mind from it’s habit of seeking imaging.

MATCHING Needed for PERFECTION of Stereo Sound

Matched output is critical for speaker manufacturers. Nearly everything is Phantom in Stereo, good best better based on matching L to R.

Keeping a singer’s voice dead center thru many octaves is no joke, see the ranges that need to be matched to prevent any lack of distinction at any frequency:

https://www.newstalk.com/news/which-singer-has-the-greatest-recorded-vocal-range-of-all-time-699955#:~:text=Based%20on%20the%20findings%2C%20Guns,Christina%20Aguilera%20and%20David%20Bowie.

ANY slight mis-matching of output, from any driver, any crossover component, any interconnect/connection, component’s internals, from time delay, from varying surface reflections will REDUCE DISTINCTION in a Stereo Image.

Some years ago I read an article about small bars or tea rooms in Japan which included quality mono audio systems.  Typically they had SET tube amplification with a single cabinet horn or other HE design speaker and vintage idler-drive turntables, such as Garrard.  Source material was either vinyl or tape.  This was more recent than the mid-80s so into the digital era.  Still, from this article, these small, commercial bars/rooms were very popular.

This is not to suggest any of us should throw out half of our systems, but there are many who still do appreciate quality mono playback.

Different strokes for different folks. If monaural floats your boat, go for it. I went in the opposite direction with my system to multi-channel and prudent application of DSP to adjust the acoustic space to fit the venue and performance of the music I'm playing.

I consider stereo to be a significant improvement over monaural with respect to reproducing a credible facsimile of the original performance, But I think multi-channel, even upmixed from 2-channel sources, is an even greater advancement over stereo in that regard, so that's my preference. Please don't throw stones.

I used to, 1980s,  play all music, even that from two channel sources, in surround sound with DSP  and seven high quality DIY speakers with all Dynaudio drivers. The more I improved the quality of my two main front speakers and their electronics the more the surround sound became a distraction instead of an improvement with everything except movies.  Now for music listening I am strictly a two channel listener.

I can think of many advantages for mono in a domestic environment. Stereo needs to be the first consideration when setting up a room if it’s to be done right, especially if you have more than one person listening ad require a broad sweet spot. Living can get in the way unless a dedicated room is a possibility. Mono is much easier to fit it to a shared environment.

If only vinyl is the source than a mono, possibly twin output, cartridge is the best place to mix it down, mono buttons on phonostages and preamps alway sound inferior, though not so much those on radio tuners. I’ve never seen a DAC with a mono switch though.