An M400 is a pretty light weight amp for bass. The MJ15015/16 outputs have very poor performance past about 5A. First thing I would do would be to go to the MJ15024/25 used in the M1.5 or better yet the MJ21193/94 .The next thing would be to upgrade the power supply rectifiers on the +/- 25V and 50V rails, drill out the pads and use 6A/400V types. Use a 40A triac selected for good fourth quadrant operation. Use as heavy of line wiring as possible and make sure the voltage is at least 120V, if it is lower buy an autoformer and get it up here. If a Carver amp can't get enough current it will increase its conduction angle until the transformer starts SMOKING! Better yet, dump it on eBay for $200 and buy something better.