REL are the easiest subs to integrate perfectly despite not having a variable phase. i had jl audio and martin logan dynamo and after months the integration still had occasional issues like peaks, midrange coloration from enclosure resonance and hard sounding lack of quality.
Note, this is for 2.2 channel music. HT is a different animal.
RELs have solid non resonating enclosures. Follow their break in and positioning guidelines and you can get an excellent integration in a few days.
RELs play low in room despite the specs and have plenty of power. Properly adjusted they offer depth, punch and do not color the midrange and are fast and non lagging.
I believe they are not so critical about phase adjustment because they are so fast and have a down firing passive radiator (my T9is) that balances the sound pulses.
finally i like the sound quality of their bass, a nice pleasing tone that is not too dry and tight or loose and flabby.