It looks like the Rel instructions advise to set the LPF to a very low value (22 or 25 Hz) and work your way up from there. Just guessing.. .agree that 25 Hz is probably way too low for most mains including M60s.

If you want to read something else and still be confused, I offer this detailed, encouraging but still confusing review :

http://stereophile.com/subwoofers/1004rel/index1.html

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8