if i was going to paint some speakers cabinets (and actually, i am thinking about it), i would completly disasemble the cabinets and sand them down for prep, i would then have them sprayed by a profesional. personally i'm thinking about taking mine to an automotive company and having them painted gloss black (the same specific raven black on my 88.5 xr4ti). i guess if you wanted them to match your walls you could roll them. i would definitly put allot of effort into the prep work though.
i don't think this is a bad idea, it could end up looking very nice in your room, but i would post about this in the DIY forum and start thinking with that mentality.