I also have M50s and a CS2 up front; M40s on sides and small B&W LM1s in rear. AVR is Onk 805.
If I had $375 to spend I would not hesitate to buy the M70s and put the M50s in the rear (they wouldn't fit on the sides).
This HSU might work: http://www.hsuresearch.com/products/stf-2.html
I think you will have a tough time finding a sub that gets down to 20Hz, is less than 15" wide and within your budget.
The Sunfire True...
Welcome to the Forum.
This is a tough question. Based on reputation alone, I would recommend all of these speakers except "MA" becuase I don't know what brand that is.
I have the Ikon6s which have great high frequency...