Sub for Proac 1SC?

Any suggestion in the <$1,000 used? room is 20*13 so faster than bigger is what I look for.
I used a Rel Stadium II for a while in my 14 X 22 room and thought it was great. Though I think subs introduce more problems than solutions, the Rel is well designed, built and melds pretty seemlessly into the mix. You can probably find a series 2 for just over 1K. Hope this helps.
I use the Vandersteen sub along with a pair of Response 2
monitors with excellent results. Rich V. has a well thought out design that intergrates nicely with Proacs.
I'm using REL Stratas III with my 1SC and it is a very good match. Don't use the included spikes unless the floor is carpeted. The crossover adjustment may take some time tweak (But you can start first by following the crossover chart from REL).

Hope this help/
Try the Proac er1 sub.