I have just received a Metal Muff that I'm running through an Epi Valve Jr. I play an ESP Viper with an EMG81 (Drop Cb) in the bridge. I have found a rhythm tone I am fairly happy with but for some reason, palm-muting sounds rather thin. I guess I would say that I want more sustain (ironic, since it would seem to defeat the purpose of palm-muting).

Since I can't afford a proper high-gain amp, is there anything I can do (besides EQ) to get thicker, fatter palm-muted sounds? I thought about changing the low E string to a heavier gauge. I have also looked into a compressor, but I don't hear of a lot of people using compressors for this purpose.

Any help appreciated.
If you don't want the palm mute to cut the sound as quick, why not lessen the pressure you're putting on the strings with said palm?