I ususally listen to the trance radio at http://www.di.fm
(I have a premium membership there, which basically jsut means commercial free and CD quality), which is also the music I enjoy mapping to.
Though post-rock is a good idea too. It's awesome to see you liking God is an Astronaut, they are an incredible band. Would be pretty awesome music to play MWLL to.
On a side note, once I get time to help out again I'd like to start setting up maps with some proper music nodes for players who like to play with the in-game music. I think it would be pretty cool to start a match and have some 3-5 min long track play when you first roll out, and one that plays at the final 5 minutes of the game or when a team runs out of tickets.
EDIT: If we could liscense music for the mod and had money to throw away, I'd always imagine Frank Klepacki's (composer of C&C music) Rocktronic album being awesome to mech to