Gain level on a REL sub

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.  

I have two RELs. Your problem is likely that this is not a punch and slam bass speaker. RELs are music audio system devices. That's why your getting the detail and textures of, say, bass strings or low organ notes or the skin of a bass drum.  This particular speaker was considered entry-level about 15 years ago. It's -6db at 25hz. You should be able to find a way to turn it up to a level that it creates boominess and sounds like it's stepping all over you speakers. To to that, turn the crossover all the way up to 120hz and you hi/low level control all the way up,  Should sound righteously horrible, but you'll hear it. Are you connected with the Neutrik Speakon connector? Check to make sure you haven't accidentally connected the black and yellow to the same speaker output on your amp.

Getting another REL will not give you the punch and slam. It's not how they are designed. I'd think Rythmik would be a cost effective solution for you. I've not heard one but do understand what they are designing their subs to do. I'd expect it to provide all of the punch and slam you could want, tho you will surrender some of that musicality you're appreciating. If this is a music system I'd suggest considering this as an evolution of your ear. If it's a home theater system then it's the wrong speaker. If your system does double duty then you've a priorities decision to make.

Good Luck!