which is more musical? A REL or a SUMIKO subwoofer for two channel only?

Excellent choice....when it comes to music, REL is the way to go! 

Congrats on the G2's!

I was turned on to RELs some years ago by a close friend, and immediately found a used Q150e (150 watt front firing 10") on Ebay for 200 bucks...perfect but needed a new grill cloth (had the frame, easy fix)...GREAT sub that seriously transformed music in my Hacienda. A couple of years later added a Q108MKII (100 watts 8" downfiring) also for 200 bucks and that's that...seriously well made subs, great sound making me a "more than one sub" for home audio believer.

Happy with my REL S3 SHO. Saving for a second one. Used, as they are replaced by the new S series which is a little more expensive. Very fast and tight when needed. You might look into 4 smaller ones like the t7 as an option.

I don't think RELs have to match...mine don't particularly although they're part of the same era...look around for ANY used REL in great shape and add it to whatever you have...you will be happy.