Recently bought a pair of M3's. Absolutely love them. As does my wife, which is a good thing. Always been a two channel guy since college in the early and mid eighties. Currently running the M3's through a Carver 400a cube amp. 201 watts per channel and they sound and perform excellent. Thinking of moving to 5.1 in the future and am looking to get 2 M2's, a vp100 center channel and an EP 350 Sub. Not sure about a receiver yet, possibly a Cambridge Audio 540. Small living room, about 1800 cu. ft. Any suggestions on this combo?

I have the Cambridge Audio Azur 540R receiver. It is a nice little unit. Certainly different from the more common brands. I bought it for its video switching ability primarily but soon it may be headed upstairs for a bedroom system or for sale. I haven't yet decided.
It does not power my main speakers, Tannoy Definition 700s. They are powered by an Anthem amp, but the Cambridge acts as the preamp section.
All in all it has been a good unit.

The QS surrounds are worth the extra money, IMO.

Am considering the QS 4 surrounds, but not sure about mounting them. Trying to save a little coin in the process of putting things altogether. Any suggestions on another AVR to power this combo in the $400 to $600 range.

Merch, if your setup is going to be 5.1 without back surround speakers it probably would be a better idea to use QS4s for the surrounds so that their very wide dispersion can better handle the surround duties for the entire room.

As to a receiver, unquestionably the 540R is very fine, but it does seem to be a bit pricey. Units available for around $300-$400 from Denon, Onkyo, Pioneer, Yamaha, etc., might be more cost effective.

Edit: well, if you're willing to go as high as $600, a great buy at this point is the factory refurb Onkyo 705 for about $530 plus shipping from Accessories4less .

JohnK, You are probably right. Just new to this home theater stuff and have read numerous reviews about all of the receivers you mention. As I said, I am just starting out and don't want to get too deep into the big money as to keep the spousal unit happy and supportive of my indulgence. Can get the Cambridge for $499 right now, but would appreciate opinions on the other brands. Have a friend who deals in Denon and perhaps could spring a good deal.

The Cambridge unit can be bought for $400 or less in good used condition as well.

Chesseroo, I have seen a demo unit for sale less than $400 but not sure about what condition or warranty on the unit. Audio Advisor is the company.

I got mine on Audiogon over a year ago for about $400 with a transferred warranty. The unit wasn't even a year old. Brand new they sell for near 1k at some stores.

To answer that last questions Denons are always a good choice. You will find many fond words for them in this and other forums. I personally can't compare mine to the others in your choice range but just wanted to answer your question that it would be a safe bet.