z640 vs mm-505

Has anyone had both of these speakers and/or listened to both and if so what do you recommend? I have the Monsoons atm. They are nice and all, but the oldest part of my system and I'm looking to upgrade them. Do the z640's sound any better? more bass? I know we're comparing $65 speakers to $170 speakers, but I'm not that all impressed with the monsoons. Their low volume sound is completely owned by a pair of 2.1 altec lansings, decent mid level volume, high level volume I guess is pretty decent for some computer speakers, but doesn't seem crisp enough. Would it be an actual downgrade to goto the z640's? If I don't get those just gonna wait for the klipsch 5.1s, or the z680's.

Yeah, it would be a downgrade to get the Z640's with regards to high/midrange response. The Z640's might produce alittle more bass. I'd suggest getting the Megaworks 550THX, if you don't need an external decoder.