Gain level on a REL sub

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.

My experience in a small professional office (11X15X8 acoustic tile drop ceiling) I have two T/Zero MkIII, and at home I have settled on a T/9. I find that the bass is subtle, just like I want it. Yes, I have adjusted up to hear booming bass. But that's not what I believe a REL sub is intended for. Their HT (home theater) series I understand does that. But I find my RELs subtly complement my setup. But that's just me.  I don't want boomy. 

What @larryincmh  read in Rel literature may be your issue. I've had a number of subs over the years, low level inputs with dsp, I always heard these subs as separate from the main speakers, drew attention to themselves. My Rel's first sub I hear simply as extensions of main speakers, subs are completely invisible. Now I can turn up volume on the Rels and get much more sense of separate subs so volume, this with only a single hot wire connected, with both + and - connected even more excess. My take is either you have issue with connection, or more likely Rel subs may not be for you, Some people like to hear their subs, IMO this can be boom or excessively articulate bass depending on sub setup, can also be excessive impact and/or volume at certain frequencies, all these are artifacts of non coherent subs. On the other hand some like these artifacts in which case Rel not for them, hear plenty from those not liking Rel house sound.

Correct, most REL especially any with Speakon connections are meant to be an extension of the speaker.  They call it a Sub bass system.  With careful tuning it does no harm to the Mid bass and mids.  You're only supposed to hear that low when called for.   For home theater it's a different story.