Home theater system with ProAc Response 1s?

Start with the sub, which should be the compact Infinity Cascade™ Model Twelve powered subwoofer features a 300-watt amplifier to deliver deep, powerful bass with dramatically reduced distortion for an incredibly lifelike sound. The Cascade Model Twelve also includes Infinity's proprietary Room Adaptive Bass Optimization System (R.A.B.O.S.) and test measurement kit, which allows you to optimize low-frequency performance to match your unique living environment. Refurb units cost about $800.

Speakers, look for used 1's, assuming you are happy with their sound. I have three pairs of ProAc's, all original Tablettes, including a EBT unit which I use for center. They sound great. I use dual NHT subs which are equalized using the RABOS system informaiton with a Technics 9060 equalizer. I don't have a receiver/processor, only a processor which I feed all the mono block amps.

I strongly suggest an initial Infinity Cascade™ Model Twelve sub. I have friends that have this unit and all are please and I think it sounds great in the different room's they have. After you get one sub, think about moving up to dual subs. Two reasonable priced subs beat one anyday. The latter fact has been proven in many double blind tests.

Go to the Emotiva web site and get their UMC-1 pre/processor (if it ever comes out) and their XPA-5 five channel amp. Both for $1500.00 now spend at least $1200.00 on a sub woofer. Your ProAcs will love the Emotiva stuff.

I agree with Bucanero that your budget will not give you a system worthy of your ProAcs. One key-get the ProAc center channel speaker to match your 1s pair so that the fronts all match sonically. (I actually have one I can sell you, or else it's going up on Audiogon soon). Subwoofer and rear surrounds need not match.

While I can't recommend strong choices based on not knowing anything of your room, listening habbits, seating arrangements, etc, I'll blanketly add that - if you do choise to build a system around a system with a pair of audiophile 2 way mini monitors - you really need to do the matching center(did they ever make one?), and another pair of the same in the rears. If you don't expect nowhere near coherent hi-fidelity sound. Basically, you might as well do home theater in a box, because the system is not going to sound good. It's just not. So, matching speaker all around, or just do the two speakers and a sub (forget the surround, and do 2.1).
Receiver? Look used high current Harmon AVR354 (latest codecs and processing, plus superb sound), or minimum of avr254 (run as small on receiver, regardless), and any good sub you can find that fits your room, offers descent reviews, and set it up well. I'll add here that 12"-18" subs for large rooms, and minimum 12"-10" powered subs for medium to small rooms, a bar minimum for ht/music - as a guide post.
If budget is a strong hold here, I'd look at avr254 (you can add an amp later as budget allows), which will free up money, still sound superb for budget receiver (under $300 referbs on ebay)