It's the source material, as you have said you have found some SACD's that sound pretty good. I have burned all my music in flac and stream them to my 3808 and only the poorly recorded cds now still play badly. MP3s and Axiom M22/M60/M80 do not get along very well. What bit rate are you MP3s? I try to keep everything that I haven't ripped from cds, to at least 256k to keep the sound quality as close to original as possible.
I do know Axiom has for some owners, sent out resistors for the tweeters to tame some shrillness. They are very easy to install, maybe try giving them a call to see if you can get some, it may help.
