Do you have a dual mono amp? If so, the wiring for the REL subs in different in these circumstances. With the normally recommended speak-on connection, you end up bridging the two channels so that both speakers are getting both channels. This may result in the problems regarding the clean sound you are reporting.
I have run 2 rel subs and also one rel sub both the way you have it set and also connected to both the right and left channels.
I have run 2 rel subs and also one rel sub both the way you have it set and also connected to both the right and left channels.