if you havent already, try pluggin the boss into the CD in phono sockets.. it bypasses the powerblocks speaker sim and stuff... means you have no control over the power block at all.. but you can control from the boss... might need a stereo jack to phonos cable... im not entirely sure what outs on your specific boss to use..
its just an idea anyway.. its ment to improve the sound anyway for running multi fx or modelers in