Isn't the idea of using subs for full range speakers twofold? First, if needed, to give the speaker the bottom most octave (which we really don't hear as much as we feel), and second, and I think more importantly, especially when used in pairs, to deal/cancel room nodes/resonances at the difficult bottom frequencies that could be smearing the mids on up?
If you take the time to optimize the Alexia to the room, I don't think the Alexia is in need of additional bass (the first reason). I'm not 100% sure as I don't have my speakers yet. But if the Alexia/room interaction is such that you have low end challenges they might be handled by adding a pair of subs.
Mike, I would wait until all the acoustic treatments are in, the power is cleaned up and the Stillpoints are on. Then use your ears and maybe another set of measurements to determine if you need them (want is another question). Just my $.02.
Before the sub is broken in you should try and get the timing/phasing in line. Not sure if or how a graph would show that. What I mean is you DO NOT want the sub leading or lagging the main speakers. I think if that isn't set up properly most folks who don't care for subs this would be a very valid reason. I've only used my ears and times when I thought I had it right I really didn't. One trick that works for me is to set the crossover a little on the high side along with the gain. I find with the sub being somewhat pronounced it's a little easier to hear if it's leading or lagging the main speakers. Then readjust the crossover point. The gain is a little more tricky. Listen to many different types of music before making a final decision. What may sound right for music with little sub bass is most likely too much for the music that really goes to the bottom octave.
