So let me preface this by saying I am not a bassist. I'm not even a guitarist, I'm actually a drummer. But I also build pedals, so that pushes a lot of my guitar/bass purchases. I only recently found out how intensely bassist love their pedals, so I've been working on a bunch more designs, but it's made me realize that my bass rig is lacking, and it seems like a shame to test things on a sub-par rig. I don't know when it will happen, but at the very least I want to upgrade my head to a big-boy amp. So right now I have a GK Backline 600 head running through a Peavey 210 cab. It sounds fine, but I know it could sound a lot better. So the golden question: If I was your friend that knew nothing about bass but had $600 or so to spend on a head (new or used) what would you recommend?