Best speakers under $500? Fire away!

Your rebuilt 3020 should not have any trouble driving 6ohms 86db as you listen to normal levels. The 3020 was renown to drive difficult speaker loads. That’s what made it a best seller. I have had the 315 driving such loads and it was just fine (before the pre-amp section started to short... but that one was a cheap made in china).
I would definitely not limit my search by speaker efficiency if the goal is not too reach the highest db possible with the given amp. But I agree, the 3020 would be a bad choice to drive MMGs or, given the specs and reviews, the Elac.
Also, keep in mind that you can try for 30 days and return if it does not fit...

B&W 685 and Monitor Audio Bronze 2 are safe bet very easy speakers to drive with excellent sound, The Infinity are very medicore speakers I wouldn’t touch them at all .

I can catch a great deal on Monitor Audio Bronze 1 speakers.  Is there a noticeable upgrade in the 2's?

The Bass in the 2’s is more deap and tight, better soundstage as well .However the 1’s are very good  too if you can’t afford the 2’s.

you're at the point 34dean where it becomes personal preference and system - many good suggestions - some like Infinity some B&W and some dislike them...you have a good amp - to some it is underpowered, to some it is ok...