Well, I've heard the klipsch, both logitechs, and monsoons.
If its music youre primarily using them for, monsoons are the way to go. They're theyre head and shoulders above the rest in this respect. Theyre the best when it comes to sound clarity/quality (I use the MM-1000's).
If its pure power you want, 560s are the way to go. The bass often times overpowers everything else and the lower midrange is... well, somewhat dissapointing.
540's are the 4 speaker version of the 340 which i also own (for my xbox).
Theyre well rounded, but the midrange is lacking, the sub is a little overpowering and doesnt extend as deep as the other subs. A great value, but its is apparent that corners were cut when making these.
As for the klipsch, they have a great sound, great power, but their price makes the alternatives all the more attractive. Sound quality-wise, theyre not as clean as the monsoons, and they lack the power of the 560's. Theyre a very good compromise between the two, but their cost isnt too appealing.
I chose the monsoons because they had the bass extension i wanted, the clarity i wanted, and had 'enough' power for me. I've never turned them up more than 1/2 way.
I'd say if you go with the monsoons or the logitechs (560's), you'll have a very nice setup that you'll be happy with. The 540's will save you some money, but you may not be too happy with them.
Hope this helps.