Trim pot
http://t2.gstatic.com/images?q=tbn:ANd9GcRmi6JLhuh4qyroLnjizspooqjVgrUUCIOZRkzRqMUsFNt7-6Zss7Ip-6-5
I assume turning pot clock wise increases out put??
Don't remember, and too lazy to take off the plate.... but, take your control plate off, plug your bass into your amp, grab a small screwdriver, and start twisting the the pot back-and-forth to see which way is louder or softer. How easy is that?
That way you'll be able to dial in the sound/tone/volume that sounds good to you. go for it!
) to find out