Gotta agree with Tharkan on getting 2 600s or if budget is an issue 2 500s. Or get 2 of each like me if you can't decide.

One technical problem which can compromise your sound quality will be using different subs. Trying to implement 2 different subs will guarantee you phase problems resulting in cancellation and peaks galore and difficulty in integration with the mains. Whichever route you go stay with the same sub.

I was concerned enough about this problem that I asked Ian and Tom to run sweeps on a 500 and 600 combined in the anechoic chamber 2 weeks ago. Those 2 subs are close enough in design that phase was not a problem and as I had hoped their combined response was additive with no cancellations. The 350 on the other hand is enough of a different design that phase integrity with a 500 or 600 may not be preserved.

John