I am in a similar position with the gain control near to max on my REL S812. The issue for me is that I am using high level and my speakers are very efficient (Avantgarde Duo 107dB/W). So there is very little voltage going to the REL.
In my situation it is ok because I have the REL situated just behind my listening position. If the sub were further away from the listening position then I would like more gain. I am aware of another Avantgarde/REL subs owner with a similar issue.
I had an SVS SB3000 sub prior to this and faced a similar issue.
I haven't tried the low level output from the pre amp. I'm not sure if this would make much difference as I only need a small amount on the volume control to drive the speakers loud.