What works best for attaching a preamp in the control cavity? I'm about to install a new Bartolini preamp and I realized I never thought about how to do this.

I usually use velcro

Or double face foam tape.

Double faced foam tape is what I've always used. Actually I should use some to secure my MM preamp that's floating around in that cavity.

+1 on double-sided mounting tape. you could also use a dollop of 5-minute binary-agent epoxy. both can be readily located at you local Home Despot.

+1 on double-sided mounting tape. you could also use a dollop of 5-minute binary-agent epoxy. both can be readily located at you local Home Despot.

The epoxy approach is a tad permanent but on the other hand that preamp won't be moving around.

True. But I absolutely HATE having things inside my bass resonate--especially when the noise crosses over into the signal path.

I use a dab of hot melt glue.