Gain level on a REL sub

+1 it's not the sub, it's the placement.  I've got at least one "suckout" location in my room that completely cancels the bass; sounds like you've already found yours.  Get one sub properly set up before you consider a second.

I have been going through the subwoofer research myself as I consider moving from a Paradigm sub that is a HT design, but not super musical.  The REL are ones I am considering. On REL’s page and elsewhere though I see cautions that when going to a more musical and precise sub than one for home theater use one may find themselves disappointed at the lack of boom.  The notes will be more accurate but the punch perhaps less. Maybe you are experiencing this. 

Need to tweak crossover 

I have a few RELs (2 in my main hifi rig) and even using various amps and speakers there's never been a gain issue. I use the recommended "high level" (from speaker outs) system and it works great. Since I've never tried the line level inputs on the RELs I can only imagine that's the problem, or not. It could be a curse or a bad REL or something to do with the relative humidity. Note that my RELs are all from the old "Q" series and continue to work perfectly, and were purchased used el cheapo.

something not right.

I sold a REL S/510 to a guy and when he connected it to his Amp (an AYRE i think) he could barely hear the Sub. After calling REL they finally advised him to switch the connection to the Amp (he was using high level) as soon as he did that "BOOM" huge bass.

regardless, you should be able to hear Too much bass with the sub turned up.