More power?

Based on your statement that your room is a very open loft, what you require is a subwoofer. While they don't go down into the 20Hz range, the Virgo is not lacking in bass when used in a reasonable sized room. In larger spaces they simply cannot pump out the volume with their somewhat smallish woofers. Would recommend either the Audio Physics models (Luna or Rhea) or the better REL units (Stadium, Stentor or Studio). If you can place the subwoofer in a corner, you will have all the bass you've ever dreamed about. BTW, two subs are better than 1.

And if I can't use two subwoofers, will using just one (off to one side) have an effect on the stereo imaging?

The recommendation for 2 subs is not related to stereo performance. 2 subs, if properly set up, will provide smoother bass with greater dynamic headroom. A single sub, again assuming exacting setup, will enhance, not impair the soundstage performance. Much of the soundstage information is derived from low frequency hall sound which only a full range speaker or a subwoofer can provide. Regarding using a single sub and imaging, if you set the crossover correctly, stereo imaging will not be impaired. This assumes that the sub is not too far to one side. Either the Audio Physic or the REL models will be easy to integrate into your system. Good luck!

I recently went from a CJ 11 to 8a's to drive my Martin Logan ReQuests. The difference was huge. Most noticeable was a big improvement in the bottom end. Try to find some 12's that you can try at home. I think you're efforts will be rewarded. Gook luck.

I am in full agreement with Onhwy61. It really does not make a difference how much more power you feed the Virgos, the subwoofer(s) is the only solution. I lived in a huge loft and know exactly what you are going through.