sub/sub amp thoughts?

I have a pair of Gallo ref 3.1s, and I would like to extend the bass. Of course, Gallo makes a SA to drive the second voice coils - I 'm looking at one in my system right now!
But I'm driving these things with a very high quality signal, ending with the massive Roque Zeus. The speakers sound glorius, and the bass with this combo is very tight (I know it's a tube amp, but I think the size has a lot to do with the control of the woofer). I add in the Gallo SA, which is a pretty inexpensive piece, and the sound goes downhill. Basically, even though it's SS, it sounds quite a bit slower than the Zeus, and when I have them both running the result is the bass is muddier. Deeper yeah, but not nearly as tight.
So, I'm thinking adding in a stand alone sub, or maybe a higher quality sub amp to drive the second woofer coils - if that's even a possibility.
Your input Please!
Also, the stand alone sub will be about 15 feet away from the rest of my equipment. Does anyone make one with XLR line level inputs? That's what balanced is really intended for - long runs, is it not?