I want speakers that are designed to excel at mono, not just have the feature. I prefer the sound of mono to stereo. I was looking at vintage mono speakers and they cost a fortune so I am just worried if they even make speakers that are tuned for it anymore.
There is no such thing as a speaker designed for mono.
You need to understand a bit of history. In the pre stereo era there was one channel only.
This was reproduced by a single speaker.
With the advent of stereo there were basically two distinct microphone techniques, intensity (coincident) and phase difference (spaced) techniques. Over the years the phase difference spaced technique has become by far the commonest, driven by the pop culture.
Either way, a stereo recording is not equivalent to a mono recording played over a single or even two speakers for that matter. This is especially true for phase difference recordings.
Back in the late 50s and sixties, there was dual inventory, partly for this reason, and partly because there were a lot of users who had mono cartridges that could not play stereo recordings.
However the mono recordings were made from different microphones and a different mix, including the early Beetles recordings.
After stereo reproducers became common place mono discs were dropped.
Now there is a version of intensity stereo called M-S matrix, where both mono and stereo reproduction are optimal. However this technique is seldom used and few engineers understand it, or how to go about it. However when I made outside radio broadcasts, I almost always used this technique, as it is a very good technique and listeners who had single speaker radios, got a perfect mono rendition.
So your issue that speakers are not tuned for mono is not relevant. The problem is that most recordings are miked and mixed for stereo, and now more than two channels often times now.
When I play historic mono recordings I just play them form my center speaker. They do sound better that way, than reproduced over two spaced speakers.
And by the way if you place two speakers side by side, there will be cancellations and reinforcements that will be dependent on wavelength and speaker spacing. This is the essence of comb filtering.
So whether you like to hear it or not, the vast majority of recordings made over the last half century or more, will sound a lot better played back as stereo and playing them mono is suboptimal.