Dear Russb: The main target of the REL subs is to augment the performance on FULL RANGE SPEAKERS ( you can read it in the page #18 of the REL manual ). Your ML aren't FRS, that's why you are looking the integration of two subwoofers. The REL subs aren't, really, for a true stereo integration, normally are designed for to run in mono way.
The own ML subs and Velodyne's can be very good alternatives.
If you want really to have a quality sound reproduction improvement from your audio system my advise is that that subs integration should be in a TRUE STEREO WAY.
Please read carefully these links about:
http://forum.audiogon.com/cgi-bin/fr.pl?eanlg&1117893153&openflup&27&4#27
http://forum.audiogon.com/cgi-bin/fr.pl?eanlg&1117893153&openflup&31&4#31
http://forum.audiogon.com/cgi-bin/fr.pl?eanlg&1117893153&openflup&4&4#4
Regards and enjoy the music.
Raul.