Proac 3.8 = Proac 50 Sig + Proac ER1 Sub?

IMHO, better to go for a floorstander from the same company. Definitely no integration issues. Moreover, I think Proac makes their subs to go with their HT range.

I am not a fan of bookshelf speakers. Unless you need to put them, as the name implies, in a bookshelf! Most of the people put the bookshelf/monitor speakers on a stand so I do not understand the logic (unless its for aesthetic purpose or you do not like the extra bass extension of a floorstander). Its taking up the space anyway so might as well get the floorstander designed as a full range.
Just my opinion.

Minimonitors can often out image floor standers because of that is in their nature. It i believe has alot to do with the small cabinet size and the nature of the resonances from a small "box." In this sense one might argue to keep the minimonitors and add a high quality sub. This way you maintain the imaging you love and get the lower frequencies your after from the sub.
If you can get the sub to integrate well with both the room and the minimonitor to produce a seamless frequency response then your good to go! This way you have potentially better imaging than you would from floorstanders and the low bass as well. Perhaps this set up is more for someone who puts imaging very high on his sonic "virtues" list. That being said a well braced floor stander of course can compete well with this set up too, of course. The way the space is allocated with each method will differ.

I've owned 1sc's, Response 1.5's and now the 3.8's. I don't really think they sound very similar at all through the mids and highs. The 1sc's and 3.8's don't share the same tweeter or midbass so really I can't see how they could follow a "house" sound.
The 3.8's throw a HUGE soundstage with increadible bass but need quite a large room to sound their best. A room that I unfortunately am loosing :-(
Can I go back to a monitor and give up that top to bottom homogeneous sound? I doubt it...
Oops, sorry got a little off topic. I can't really see the sub and monitors giving you what the 3.8's will do as they dont' sound that much alike anyway. However you may still enjoy those monitors and sub, remember though that your smaller room may not be subwoofer friendly either...

In response to your original question. Try it. Get a sub and see how well you succeed at integrated it to your room and to your tablettes. Perhaps going with a proac sub will help in the matching between the sonic signature of the tablettes and the sonic signature of the sub being that they are from the same company. Not a guarantee but a good bet. If the sub is tunable. Try and dial it in as best you can to both the room and minimonitors. Experiment with different spots for the sub.