I don't know the REL G series subs and I can't speak specifically to the phase issue that you're experiencing. However, REL subs have not generally fared well on the deep bass/clean output tests that I've seen - which could explain the LFE bottoming. This may well be by design, in that REL subs have (rightly or wrongly) long been touted for use in music, rather than cinema applications.
Compare the data on these (older model) RELs to a sub like the SVS models, which are clearly optimized for LFE:
http://www.hometheatershack.com/forums/subwoofer-tests-archived/6015-index-subwoofer-tests-manufacturer-model.html
Your subs are newer and may be designed quite differently, so a grain of salt may be required in the evaluation here.
Good Luck.
Marty
Compare the data on these (older model) RELs to a sub like the SVS models, which are clearly optimized for LFE:
http://www.hometheatershack.com/forums/subwoofer-tests-archived/6015-index-subwoofer-tests-manufacturer-model.html
Your subs are newer and may be designed quite differently, so a grain of salt may be required in the evaluation here.
Good Luck.
Marty