Magnepan 20.1 versus Infinity IRS Beta

If you are considering each option why wouldn't you also consider the new 20.7?
This is not an answer to your question and I don't think the new 20.7 will extend the bass further.
But it seems to me you (especially since you are at close to the same price point) that you might want to consider the 20.7 instead of the 20.1

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Because you don't mention the 20.7, price must be a concern. You can buy a set of Betas, if you can find them for around $5-6k and Maggies around $7-9k. I would suggest getting the Maggies and a pair of good subs. The Betas are great speakers, but they are also 24 years old. Parts and repair will be a hassle with the Betas. Going the Maggie/sub route will be more expensive, but I believe that route will be more practical...plus you will get excellent sound. You need a big room for the 20.1, you'll need a huge room for the Betas.
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I had owned since new a pair of Infinity RSIIa. (the next smaller EMIM equiped system from the Beta.)
I now own Magnepan 3.6

I would say either will allow really great mids and highs.
The Infinity will do bass better. If you want 'slam' them the Infinities will be better.
(IMO the upper range will be a tossup, the mids also, the Infinity will have a bigger discrepancy between the mids and lows than the maggies.

And I agree the Infinity are getting old. The biggest item are the EMIMs inner films, they are no longer available, and i sold a bunch of EMIMs for $200 EACH (I gutted my infinity speakers and made more money in parts than selling whole.)
Some company has (recently) made a replacement inner critical part for the EMIMs and EMITS. And if you get the Infinity, you should definitely get a full set of replacement inner foil parts for the EMIM and EMIT speakers.

Either set of speakers can really sound great.
I would say the Magnepan will offer a more seamless presentation. a better soundstage too. The Infinity will ROCK if you want that, in a way the Maggies never can

Tough call.