From my own experience, I found that the 3A Sig's, when combined with a single 2Wq sub in a small to medium-sized room, can be driven with as little as 50 wpc, PROVIDED you don't listen at high SPL's. That said, I have found that my 3A Sig's work better for both audio and HT with at an amp that offers a clean 100-150 wpc (3-4 db increase, which improves the headroom dynamics).
My current system has 3A Sig's and a stereo pair of 2Wq's (which I find far superior to a single sub, not just for higher SPL's, but also for better audio quality). My 3A Sig's are powered by a Bryston 4B-ST amp (about 300 wpc), but I am confident that a 150 wpc amp would be more than sufficient in my medium-sized room. If you add a stereo pair of 2Wq's subs, and you don't listen at ear-bleed levels, then 100-150 wpc will normally be adequate to drive the 3A Sig's.
A closing thought about the 2Wq subs. I upgraded the crossovers for my 2Wq's to the X-overs made for the Vandy Model 5 speakers. These X-overs aren't cheap -- $600 for a single-ended pair (separate, stainless steel boxes for each channel) -- but they are virtually seamless, and a decided improvement over the standard fixed value crossover. The Model 5 crossovers have first-rate internal parts, and have dip switches that allow you to change the values should you get a new amp. Admittedly the Model 5 X-overs are icing on the cake, but certainly worth considering if the rest of your system merits them.
As I have commented before in this forum about the 3A Sig's when combined with the 2Wq subs, you not only get higher SPL's without distortion, but the mid-range clarity improves as well (Richard Vandersteen says this is due to the decreased IM distortion of the 3A Sig's drivers). The improved mid-range is a very nice "plus" from adding the 2Wq subs, but there is an "unintended consequence": a tendency to listen at higher levels because everything sounds so clean. This may necessitate a bit more amplifier power, so if you plan to add either 1 or 2 Vandy subs, you might be best advised to use an amp with a bit more power than the minimum you think you need.