2nd powered subwoofer

Ideally yes. Ideally the sub should match the speakers - not only should it be designed with same Q or damped response to preserve timbre but also to get better phase accuracy through the crossover between mains and sub. If it is mainly for movies then I would not worry about this though - just use the matching sub for music and both for HT.

I use a REL Stadium 2 on one channel and a Stadium 3 on the other with good results. A smaller sub from B&W might be the best choice. I would check used ones. I bought both mine used.

Not unless the room is perfectly symetrical in every aspect including furniture, window treatments, etc. Really, I challenge anyone to honestly hear a difference when mixing two decent powered subs in the real world. Though I would agree that it is easy to hear differences between 2 dissimilar regular speakers , and even two of the same model speakers. As for phase, thats easy to test by flipping the phase switch on one of the subs, or reversing the input wires on one if no phase switch. Just make sure you are using a test CD phase test track first for a control and the subs are in phase with your other speakers. May be able to hear the differences if any between the subs with the main speakers off. Use a sound pressure meter to balance the subs volume from your sweet spot.

Well, after much researching/studying up, taking your advice, and stopping at a few hi-fi stores for their opinions, I made my purchase today of a 2nd powered sub from a highly reputable B&W/Rotel dealer. It matches my existing sub but is just the newer version (B&W ASW610 with an Audioquest interconnect). I finished getting it all set up and am playing now. My first reaction: ahhh yeah !! Still tweaking and playing and allowing it to break in. I don't think subs need that much break in though.