Gain level on a REL sub

Thanks @highend64 .  Well I am stumped. While I wholeheartedly agree one sub is not as good as two, you still should get good volume and punch out of one.   For High level, you are tapping out of the amplifier outputs?  How are you connecting this as you have one sub and four amplifier outputs?  Have you tried just connecting to L and R outputs of one channel or one amp? (using  the Speakon connector and Red / Black wires?).

My REL r305 is very similar and I’m running it speaker level from a 40 watt Integrated amp and I only have the volume 5 clicks up from zero and a low crossover and it is plenty loud and keeps up with a pair of 88dB sens speakers easily.   Just a few clicks up and it is way too loud.  12x 14 room , well treated 

it almost sounds like it’s not getting enough signal from the amp   For troubleshooting purposes I would strip it down to its most basic elements, no EQ    Just the amp, speakers, sub , and a good line level source  

You could be sitting in a null. A larger sub will still have the null.  Also, move the sub around. Including into the corner.

Try the sub right behind your seat.

Small rooms can be problamatic.

@OP. I know you say the situation was different with a previous sub, but perhaps you are expecting too much of the Rel. There's only so much a sub can do when the main speakers are limited in frequency extension.

But even given that, there is no way that any Rel sub should sound insufficiently loud - even well below its maximum gain setting.

If you connect using the low level input you can use a set of test tones and an SPL meter to get an objective measure of the subs output. That will be of use to you in moving the sub around in case it is on a null point in the room.

+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.