I own both a Mesa 400+ and an Ampeg SVT CL, they both are amazing amps, they both have Pros and Cons. I play Professionaly in a band called Towncrier (
www.myspace.com/towncriermusic) and a bunch of cover bands that play to 300-500 people a weekend. A few of my bassist friends and i have agreed the magic combonation is Stingray 5 + Tube Head + 8x10 Cab. It also helps that i have a lot of people to help me load my gear, but for smaller gigs i use a 4x10
Mesa
Pro: Lighter than SVT, WAY more versatiltiy, 500 Watts, runs at 2 4 8 and 16 ohms
Can run both of its inputs at the same time to create a very thick round sound.
Cons: This is not set it and forget it amp, it has many tone shaping capabilties, which is also a pro.
SVT CL
Pro's: Set everything to 12 and it sounds perfect., 300 watts (less volume to get to ideal drive level)
Cons: weight, only runs at 2 and 4 ohms.
I use the mesa with my touring band, and keep the SVT and a 4x10 at home. While i love the mesa, i dig the simplicty of the SVT CL and it will always have a place here. SR5 + Tubes = Delicious
I think im rambling now...
-Cory