just to clear things up, the pioneer units have a usb slot on the front, the the main connection is the second connector on teh rear of the unit and the included 3-4' extension cord. You run the cord into your glove box, and plug something like this
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136229 into it.
Then to add music, instead of detaching your faceplate and loading 1 gig of music, you just unplug the hard drive and dump up to 250 gigs onto it. OR plug an ipod directly into it. The extra space means you can rip music at much higher, better sounding bit rates - the pioneer will decode up to 320kps WMA , as well as mp3 and aac. I just wanted to make sure you knew how the units can be used.
As for speakers, i do like the MM series a lot better than the db's, but they like a lot of power - 50watts may be a little light for them. At least the last MM series liked as close to 150w rms as you could get (i had them installed when i had my truck and noticed a big difference when i went from giving them 80w rms to 140w). The new ones might not need quite that much, but you cold always try bridging your amp, too.
I HAVE heard fantastic things about the SR series, esp if you like the smoother sound of a soft dome. The SR's use ring radiator tweeters made by VIFA.
If you like the sound of soft dome speakers, I'd agree that Focal and Diamond are probably not going to be your taste (probably along with Boston and MB Quart). A few brands that do use VERY high quality soft domes are CDT (
http://cdtaudio.com/ ) and Morel - worth taking a listen to.