my subs are rated at ~95db. i have successfully had speakers from 86db to 101db in my rig, w/the marchand xm-9. besides having separate bass/treble pots, there is a pot *at* the chosen x-over frequency that is really nice for dialing in different amps/speakers.
doug s.
Ewe--ewe listen to me!
It's very important to match the speed of the amps and the tonal characteristics of woofers and subs. Keep the ProAcs and match them to smaller subs w/ similar materials as the little woofers... 
well... this has not been my experience. or, perhaps i've yust been lucky!

i have had the following speakers in my system, all with vmps larger subs:
thiel 3.5
meret re
totem arro
nht super zero
zu druid monitor
proac tablette 2000 signature
proac tablette 8 signature
swan m1.2
gr-research criterion
gr-research diluceo
older spendors (don't remember the model #)
the subs were upgraded w/megawoofers a few years back - only the thiels & the merets were used w/the original woofs...
i have had two different amp configurations - 1st was a pair of electrocompaniet amps on the monitors, w/a pair of original adcom gfa555's on the subs. 2nd was ec's on the subs w/a mesa baron tubed amp on the monitors.
the only variation to this was when i used the flea-powered asl wave-8's on the monitors, w/the ec's on the subs. this was done w/the meret's & the swans. the swans actually sounded better w/the wave-8 than w/the mesa, due to the lean midbass characteristic of the waves, & the full midbass of the swans...
w/the aid of a spectrum analyzer & graphic equalizer, i have always been able to get a nice seamless match. smaller subs are ok w/smaller rooms. big rooms require big subs, imo, regardless of monitor/amp. the diminutive 4.5" woofers of the diminutive proac tablette reference 8 signatures mate completely seamlessly w/the vmps larger subs, & the sound eminating from this system in the quite large 26x38 room is really quite amazing - almost bizarre, really, considering how tiny the ref 8-sigs really are... i do admit, however, to preferring the merets by a nose, over the proacs in my system. they *are* a bit more dynamic, w/their 7" woofer... the meret's are still the best speaker i've had in that rig, much to my chagrin. i do not know what might be able to replace them, that i could afford...
ymmv...
doug s.