I use 2 older series (Q series, Q108MKII and Q150e) and they blend superbly with my Klipsch Heresy speakers. Note that even with the supposed 99db efficiency of the Klipsch main speakers there is a LOT of headroom left in both subs even with my relatively low output (maybe 15wpc) single ended tube power amp...the level of both subs isn't anywhere near being maxed out. I won't be buying any new RELs however as they refuse to service mine as being too old...luckily they don't need any service so far but still...REL gets my permanent boycott for that...