Brass. Copper would corrode too badly and is too soft to hold a shape. The brass looks cool and will not corrode (but will tarnish over time...a bit of Brasso will make 'em look like new again).
It's turning into a bit more of a project than I thought...the preamp has very good drive capability (500 ohms) until the tone controls are switched in, then it goes to hell (3.5K). That might be good enough for a lot of amps, but in general kinda sucks. I'm buffering the output with a Burr-Brown OPA2604. The 7100 also does not have sutable split supplies to power the opamp, so I also have to fashion a +/-18V zener regulator to power the opamp.
It'll be fine once done, but so far has taken most of the day...
Edit: The copper would oxidize, but it is not a huge problem (so I am informed), as copper oxide conducts just fine. I still use brass because 8ga solid-core copper is a pain to find.