I play mostly classic rock, a little hard rock, and some blues. My Mesa stuff consists of a Big Block 750 Head, a Powerhouse 2x15 , and a 2x10 cab. Couldn't ask for a better sound. I'm curious about the new M9 though. Looks like a monster!
I use the Black beauties on one of my basses because they have a warmer sound. The rest of my basses have Elixir. That will fix you problem with tone if you want coated strings.
You could install the i-tunes program on your new computer, and then look for this program called idump. It's free. It will load all of your songs off your ipod onto your new computer, so you could sync all you want after that. It may take a little bit to load your songs from your ipod...
I would go with a mesa bottom, since you have the head already. Try the PowerHouse 1200 bass cab. It has 1 15" with 4 10" in the same cab!! 1200 watts! This will give you some crushing lows, and bright highs!
If your bass is in tune when you hit the string open, then to check to see if your intonation is good here's what you do. On the 12th fret on each string, play that note. the sound you hear should be the same octave as when you play it open. If it is not then you have an intonation problem. If...
A Mesa Powerhouse 2X15 cab comes in either an 8 or 4 ohm unit. I have the 4 ohm. Rated for 1200 watts. Very nice sounding, and very loud with the right head.