You are a couple of days early--my VT 100 mk III shows up tomorrow! I've got the Response 2s, which are mated now with a pair of Quicksilver Silver Edn mono amps. I decided to upgrade and liked the way the ARC/ProAc combos provided resolution without sterility. I also did some net research on other's opinions and found out that ARC/ProAc seems a safe bet since I gather ProAc voices their gear with ARC hardware. I also saw an article noting that ProAc's US sales went up substantially at some point when Cary was selling some of its amps paired with the RS1s. I've never heard that combo, but that sounded like a pretty good endorsement you might want to check into. (Left me scratching my head a bit, however, 'cos the RS1s are only 84 dB efficient or so and I thought Cary produces amps that like very efficient speakers)
In this vein, I ended up with *two* VT 100 mk IIIs (long story) and, just for kicks, emailed the guy I originally bought my RS2s from, who has a *long* relationship with ProAc, and asked his opinion regarding bi-amping and the benefits of using an external x-over (my preamp is a Theta Casablanca which can be configured as an external x-over). The response (no pun intended) was to "passively bi-amp horizontally." Oh yeah, and to "get an ARC preamp."