subwoofer question......what size sub ?

Power Sound Audio have a wide variety of subwoofers with some of the most generous options such no shipping charges and a 5 year warranty.

FWIW I have used  Entec, Vandersteen, Velodyne and REL subs with 4 different series of Proac speakers.
My ears like the REL  products. They are fast and accurate, not to be confused with subs that make Top Gun(and Tom) sound great!
They have a very simple attachment method that takes the output from your amps speaker terminals. Very adjustable and I have yet to find a room that they can't be tuned to work in... YMMV.

Get a 10" woofer model and start the learning curve about room resonances and standing waves.
The Proac/REL combo can be very rewarding.
Good Hunting!

If you look at speaker driver's data, you find 8" to 10" bass  speakers perform the best in the right cabinet. An 8" in a transmission line cabinet may be better than a 10".

Cone speed is as important as frequency response; the greater the mass of the speaker cone, the higher the risk of latency. If speaker cones (tweeter/mid/bass) have different latency, sound gets out of phase.

Martin Logan's electrostatics are classic examples of this where they struggle to get bass and panel phasing correct.....personally I never liked the sound of them for this reason. Quads had poor bass, but the phasing was lovely, sorry, getting off topic.

I agree with wallyfl., Here's my two cents;
I have three REL R-528 and one REL T-9. The T-9 is used for center in a home theater set-up and one REL R-528 behind the sofa. In front for LF and RF I have Paradigm Signature S2 and just outside them are the REL R-528's. I have them connected both high level and LFE. For two channel listening, I'm always amazed at how well everything blends. I sometimes have to put my ear up to the center channel (Paradigm CC-690) to see (hear) if it's putting anything out. Again, only left and right channels are active. Then I go back to my listening position and enjoy! I believe a good quality bookshelf with a good sub sounds as good or better than any tower since towers do not have the low frequency extension you get from a sub. And the REL, by connecting it via high level, it's getting the same exact signature and the main speakers so timing is right on. The REL is undisputedly the most musical subwoofer available. You have complete control of it's volume level and crossover frequency, ultimate flexibility to fine tune to any room. They will surprise you!