Alan, Unfortunately, you do not mention the models of the other brands.
Bose.....I presume you are talking about the 3-2-1 system? This sounds OK but is not in the same class as the Denon which it is similar to. Bose are very pricey for what you get.
Denon.....I presume you are talking about the 101 or 301 system. These are a big step up in sound and build quality from the Bose. Most reviewers feel these systems give very good sound within their design limitations.
Polk.....I think I have seen an add for a new Polk 5 speakers in one enclosure like the Yamaha but not sure at all about this. Generally Polk speakers are very good quality.
Yamaha.....The YSP-1000 and its new little brother the YSP-800 are in a different design class from the Bose and Denon. They have speakers (many) covering (left, right, center and 2 surrounds) in one enclosure with its own surround electronics and amp. You've seen it so you are probably familiar with the idea. The Yamaha requires a separate powered sub (any brand will do..probably one in the $300-$600 range would be a good match). I understand the sound is very good to excellent. The YSP-1000 is fairly wide (42" ?).
M & K (Miller & Kreisel).....model MP-4512. This is a excellent brand of speakers but not as well known by most casual consumers. You might have to search for it (or Google). This model is a fairly compact, only about 4.5" H x 18.25" W x 5.5" D. It has the 5 speakers (l,r,c,2s) in one enclosure, about $650. You hook it up to a regular AV recevier. It also needs a powered subwoofer in the same price range (sub needs to cross over at 125Hz or higher). Sound is excellent. This unit has very high quality speakers and will play very loud without breaking up, don't let the small size fool you. A real best buy.
If I remember correctly the Yamaha tends to have a sweet spot that is best for optimum listening. If you sit in one general location then this should not be a problem. It also plays very loud.
The Denon, Yamaha and M & K are all different in design. The Yamaha and M & K (with subwoofer...this is a must with both systems!) are definitely superior to the Denon in sound. I'm talking here about the bass reponse, loudness and definition.
The Denon appeals to those who want a very simple system with very good sound for the price and size.
The choice between the Yamaha and M&K for me is not easy. They are very different in size and design. Both will sound very good to excellent with music as well as movies. The Yamaha will probably have a slightly better illusion of normal 5.1 sound and the M&K might be slightly better at frequency response accuracy and definition.
Hope this helps.
I'm interested in simplifying my surround sound system to eliminate the need for rear speakers. I'm quite impressed with the Yamaha YSP - 1000 and wonder how it compares with some of the other rear speakerless systems, such as Bose, Denon and Polk. I use the system for viewing HD TV and DVD's, and do not depend on it for listening to music. The system is located in a fairly large family room about 15'X 24', with the viewing area about 10' from the TV.