@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.