Whether its a vintage amp, a combination of stomp boxes or just a direct box, try to work it out before the studio clock is running. Again, there will be some experimenting here, but preparation will pay off. During the tracking sessions your job is to keep the sound glued together. Hopefully, you'll have a chance to replace the rough tracks you laid down. At this point, you can fine tune your sound, and work on the performance.
Keys are the easiest thing to prepare ahead of time.
If you have an arsenal of modules, use a mixer so you can feed the engineer a stereo output. This will assure you of having the sound you intended, and relieves the responsibility of communicating your sound verbally to the engineer. This will save a lot of time in the end. One of the biggest mistakes I have seen in the studio is the band underestimating the time necessary to cut basic tracks. If you are cutting multiple songs, try to work out a lock-out rate with the studio for several days.