For most circuits using the 6080, you can use any of the following: 6AS7, 6AS7G, 6AS7GA, 5998, 5998, 7236, 2399, 6N13, 6N13P, and 6520. Of course, check with the owner's manual or manufacturer first before dropping these tubes in. Of these, I'd say the most "colorless" would be the Sylvania 7236. On the opposite side, the RCA 6AS7G is very colored and warm.
@lextek, I don't see an issue coming from this, just make sure the connections stay the same.
Also, I was going to ask this on the Bottlehead forums but havent finished registration. I have a 6080 power tube in right now, is it easily replaceable with a 6SN7? I've read a couple things about needing to rewire, but others saying its a drop and go. If anyone can help, that would be awesome =)