The deCapo is beautifully balanced as a stand-alone. The presence thru treble is gently rolled off to balance the modest bass energy. As a stand-alone monitor, it's really, really good and IMO one of the best values out there (which is why I bought mine). When I added a subwoofer (for pretty much the same reasons stated in the OP), I loved the extra weight on the bottom end, but the sound as a whole went dull. I tweaked it for a long time, but eventually moved on.
I think I'm one of the bigger subwoofer fans on this board and I'm surely one of the bigger deCapo fans, but the combo didn't work for me. YMMV.