Have been through a progression on a number of different subs and several Rel models. Totally agree with a number of posts that Rel subs are most compatible for 2 channel and not the best choice for HT. IMO the best Rel for HT and music combined is the SE/SX series. For my setup I have a stereo pair of Rel's for music and a JL Audio sub for HT LFE.
Having said that, sufficient volume should not be a problem but will not sound great at max gain. Could be an issue with a number of potential issues already mentioned? Generally, the common mistake in setting up Rel subs is to have the X-over set too high and the gain too low. Properly set up Rel's should add major "dynamics" to the music but be totally transparent. Should not be boomy. Comment above that the presence of the sub should not be obvious until you turn it off is right on.
Good luck in the quest...