Consider the Behringer DCX2496 (XLR in/outs, but RCA/XLR adapters are easily had).
Active crossover and digital EQ functions.
Under $400 new.
I have found it transparent in a $15K system. It's EQ function has tamed a huge (+15dB w/Rat Shack analog sound meter) 50Hz room node at my listening position. Given that competing products cost 10x or so more, it's a steal.
In many cases allowing your monitors to roll off naturally at the low end when augmenting the bottom octave with sub(s) will give better results, particularly midbass imaging and especially at low/moderate volumes. The Rat Shack sound meter (admittedly imperfect)plus your ears will allow you to use the Behringer to blend your sub in a more controlled fashion, possibly allowing a sub low pass cutoff well below the 80Hz, where the sub will actually be handling things like the bass notes of male vocals (yech).