I recently purchased and XMP3 portable xm radio. I had it installed the other day. The wired it into my auxillary of my JBL stereo in a 2004 Toyota Avalon. They adjusted the volume level on the xm radio to the maximum output. The problem is my overall volume using the radio volume button doesn't get loud enough. I spoke to the installers about the issue and they haven't gotten back to me yet. My confidence in them is pretty low so I came here to see if anyone had any ideas(could I add an additional pre-amp?