I have heard the M3's and thought they sounded very nice, indeed. I own M60's and like them very much.
I have a couple of theories on your situation.
First, it certainly could be "something" with the receiver. I guess you would need to not only try more than one pair of speakers, but also perhaps get another set of ears in the room.
Second, it could be that your existing speakers have blown-out drivers. You may be perceiving the resulting distortion as shrillness.
Third, and I sincerely mean no disrespect, have you had your hearing checked lately? Some people suffer from tinnitus or other symptoms that make hearing fairly unpleasant at times. In some cases, the only time people notice these symptoms is when they are listening to something critically.
I hope you will enjoy your new speakers!
