Old and New-- adding Rel s510 or 812 Sub to heritage (older) McIntosh system-- Worth it?

Forty plus what? Ten? Fifteen?

Be good to your back. Get the small wooden $12 dolly from Harbor Freight, a 20'-30' RCA cable from Blue Jeans cable and a ground lift plug. Decoupling with the dolly may be a second surprising improvement.

In your reading any opinion that lacks a direct in room comparison of another brand its relatively worthless. A direct comparison with the -6dB "Sub-Bass System" your considering and a -3dB EQ equipped Subwoofer would provide you a valuable understanding. Afterwords you can return both of them.

Since this community is decades behind pro audio integration of low frequency you might want to use a number of manufacturers who offer home trial before committing to a used unit. yogiboy's advise could easily kick in 'all over again.'

All the best with your search.

Thank you yogiboy and bigtwin-- I agree with you both in the sense that I will only buy a Rel unit that I can listen to and then return if unsatisfied.  Your comments are very helpful in that regard.  I certainly don't want to "mess up" a good thing, but I have read some really good things about Rels for two channel audio, as you mention bigtwin.  Currently trying to decide from buying new from Rel, or a "scratch and dent" from a dealer.  Both are returnable up to 60 days.  I have to add the people at Rel have been really first class in answering questions.  Thanks again.

Borrow a Sub and see if you like it.  I have two REL S/812 and they add a lot to my system.  Let your ears guide you, not what anyone else thinks you should do.  Cheers.

It sounds like you really like the magic that you are now enjoying. Don’t ruin it adding a subwoofer. You might be taking one step back!