ProAc 1.5's add sub or trade up?

I own ProAc 1.5's and am considering adding a sub. But before I do looking for people who have experience with 1.5 & sub combination or suggestions on filling the bottom end for about the same money ($1500.00) in a single speaker.
ProAcs work very well with the vandersteen 2Wq. It helps to free them from much of the driver excursion they are now doing when trying to make deep bass (below 80hz) and the air pressure and resonance that comes along with that deep bass. They will sound faster, clearer, smoother and more effortless once the 2Wq & crossover are inserted into the system. Of course the bass will be much more extended and powerful with the sub in the system.
I have 1.5s as well. This can be a difficult thread since expectations on bass could be different. I really like this speaker and satisified with the bass as well as other qualities. Room size and placement are considerations and I am sure you have tried moving them around. I think with this speaker as well as other Proacs, you need allot of current from your amp. I think some people might be dissappointed with this speaker because they are not driving them with the right amplifier. I think Proac came out with the D15 replacement for the 1.5 because it can be marketed to a broader audience ( consistent bass performance from a broad set of equipment). I am using Rogue Audio 120 Magnums and I'll tell you the bass is tight and satisfying (ultrinear more bloom, fuller; triode tighter and more tuneful). Some thin sounding solid state gear or low power tubes will not cut for this speaker.

I am not sure of your electronics but I would look to that direction as better alternative to a sub. Again I really like this speaker.

Personally I would not advise a sub with this speaker. If for any reason (besides difficulty in integrating) for what a good sub would cost I would get another speaker or look to other electronics.
I was in same situation before but found out that speaker placement, amp and speaker wire were the key to get the most
out of these speakers. I'm using White Audio Labs/Llano Design amps with very good result. My listening room is about 20'X17'X11' and I get good bass response down to mid
30's (-6db). I did try my HT subwoofer with this speaker but
did not like the result. The good bass drum response definition and imaging were no where near compared with the 1.5s alone.
As a prior owner of the 1.5's, I would suggest that you look to speaker placement before making any further changes. Check out the excellent instruction on the Audio Physic website.
Before I bought the ProAc 1.5's I already had a Vandersteen 2wq sub in my system. All I can say is that I am very satisfied with the bass produced with this combo. I think a Vandy sub is a great foundation for almost any system. It is a music sub not a home theater one. The filter system from the Vandy sub will also lower the distortion in the mid woofer and lessen the load on your amp to. I would also like to note that if blended properly this sub never calls attention to itself. If you consider this sub make sure you talk to a dealer that knows what there talking about or call Vandersteen to insure that it is compatible with your system. Good Luck!
Thanks for all the info. I am using them in a HT set-up with a marantz sr-19 (might be some of the problem) bi-wired (shotgun) with Harmonic Tech melody speaker wire. I guess I should look at entire package before making any decision. Thanks again
Thanks - Purchased a proac sub a week ago. Thanks for all your suggestions and recommendations.