Regarding the M-Audio, I'm not sure about the CX series, but I do know that the BX series (both the 5 and the 8) have had a lot of user reports of breaking/malfunctioning, see these;
http://forums.m-audio.com/showthread.php?11926-Bx-5-monitor-problem
http://forums.m-audio.com/showthread.php?28012-Broken-BX8a-emailed-repairs-m-audio.com-no-response
http://forums.m-audio.com/showthread.php?27521-Broken-cross-over-circuit-on-BX8a-Support-difficult-to-reach
http://forums.m-audio.com/showthread.php?20795-Long-list-of-bad-BX8a-reviews..
And a whole host of others on the same forum. The problems may not translate over to the CX series, but I'd be wary...
I went through auditioning a few of the M-Audio speakers in my search as well, while I haven't heard the CX series, I thought the BX5A sounded like a tin can.
The KRK's are OK, unfortunately only the 6 is within that price range, which may or may not be enough bass depending on what you're listening to (auditioned it and found it to be somewhat inadequate), the KRK 8's cost substantially more than the Prodipes, and to be honest... I think the Prodipe's wipe the floor with the entire Rokit series, if for nothing else then the ribbon tweeters which are very accurate and pleasant sounding, it's a specific kind of sound, these are my first ribbons, and I definitely enjoy them (I'd love to build ribbon headphones
) along with the fact that there aren't many (any) alternatives in that price range if you want 8" woofers.
So, in my opinion, these really are the best bang-for-the-buck speakers for $400 if you can get them for that much. I paid substantially more in my country but all the prices are equally higher here, they are still about 20% cheaper than the KRK, etc. offerings.
All in all I'm very happy and they are a pleasure to listen to, the stereo imaging and crystal highs especially stand out for me, I can close my eyes and feel surrounded by the music, can't tell where the speakers are at all - just make sure to position them properly. I do not mix or record music but I do enjoy quality and accurate sound, I got a few weird looks from the people at the store when I said I was buying studio monitors just for music reproduction
Good sound is good sound, regardless what you use it for, and they are simply perfect for desktop use (well, if you have a large enough desk, which I need to take care of...)
Unless you're a particular basshead, position them correctly and I doubt you will find the desire for a subwoofer... if so, there is always their Pro 10S to complement them later on
If you have any questions let me know.