First time poster and owner of SP4's, just ordered through the upgrade program, haven't received them yet. I currently run MIT EXP-2.s to LSiM 707's. The 707's are new to me as well they have about 200 hrs on them so far, and seem a little laid back in sound, very full and rich but a bit subdued in the top end. I'm running a Yamaha TSR 7810 to a B&K Ref. 200.5 in a dual HT / 2ch setup. I will be receiving my Emotiva DR2 amp tommorow! Can't wait to see if putting 550 to the 707's will make a marked difference! I'm hoping these SP4s will open them up! Thoughts?