I also use a REL with Heresy III and they sound great together. You can NOT use RELs Neutrik connector wth fully differential balanced amps like ATI or many oof the amps they OEM for because they too are fully balanced.
I have a pair of PP mono tube amps and I can't get a diffinitive answer if connecting the REL cable is safe or will deliver both channels so I have just used my Veledyne SMS 1 which I prefer because I can experiment with low pass, slope, and manual or auto EQ. I have the R-305, it older but still sounds great. I have it hooked to the LFE so the sub is not using it's x-over
The biggest improvement I can suggest is a isloation platform like the Auralex Sub Dude. Unbelievable improvement in a room with hardwood .
I have a pair of PP mono tube amps and I can't get a diffinitive answer if connecting the REL cable is safe or will deliver both channels so I have just used my Veledyne SMS 1 which I prefer because I can experiment with low pass, slope, and manual or auto EQ. I have the R-305, it older but still sounds great. I have it hooked to the LFE so the sub is not using it's x-over
The biggest improvement I can suggest is a isloation platform like the Auralex Sub Dude. Unbelievable improvement in a room with hardwood .