Thank you all for the very thoughtful and helpful advice.
@needfreestuff I will definitely heed your advice to take my time to investigate main speakers that might be an even better match for the RELs. I have owned the Platinum speakers since before the turn of the century. I can certainly wait longer before considering another upgrade. You helped me realize that I should take the time needed to deeply consider what I enjoy from this pairing and, if anything, what I find lacking that an upgrade might improve. If it can be beat, from the reviews I read and listened to thanks to your recommendation, the TAD CE1 may be it, but I'll need to wait for a gently used one to fall into my budget. The Graham 8/1 had less reviews, but also seem a worthy consideration. Thanks again for making me aware of two options I had not previously come across in my preliminary investigations.
@m-db on your recommendation I contacted REL. They, and the McIntosh MEN220 forums on the web, provided recommendations that brought further depth, height, and punch to the soundstage. The Platinum speakers have improved by using the active crossover. Thank you for a suggestion that had great impact on my enjoyment of the now more finely integrated pairing.
@audiojan your recommendation prompted some further thinking on the MEN220 setup. Currently, I have the Platinum speakers connected to the high level input and the REL subs connected to the low level input. Am I correct in understanding that you were suggesting a different connection? I have moved the subs around and found a sweet spot that interestingly seems quite similar to a Youtube review of the TAD CE1 referenced above which also had a pair of s/510 subs in what looked to be the same locations as I found to be best.
Thank you all for your advice. I am a newbie to posting on this forum. I am so glad that I took the plunge to seek out your wise counsel.