Gain level on a REL sub

I know this is basic, and I know we all know this, but MAKE SURE your  main speakers are in phase.  I bought a new integrated to take to my vaca home, and pulled out two cheap pairs of small speakers, to try it and compare.   I accidentally wired the second set backwards, with one speaker's wires reversed.  My o my did they sound bad.  I finally figured out what I did (embarrassingly), switched the wires, and the bass, image and everything improved.

@jji666 Yes, i am aware of them being different  sounding subs. They both are great subs but in different perspective means. As far as the far as the gain setting, it is not maxed out.  It happens to be at a little before 3/4. 

@fastfreight I have color-coded speaker cables. I would have to be color blind if that was the case.

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.

I'm using modded Klipschorns, 103db have only one of the signal cables connected to each of my 300B monoblocks,  REL volume control only two or three clicks up from minimum. Two signal cables connected massively excessive volume. This with two REL TX9 in 13 W x 35L room, diy custom build 6' L sub cables, Speakon connectors single signal wire and ground to each sub and amp, ground wired to chassis screws not negative speaker binding post, bare wires at amp end.