The Phat Phuk B has an internal gain trim control, the main knob is just output volume. Also check out the Earthquaker Devices Speaker Cranker. Another one knob boost, this time the knob controls the pre gain only. The breakup is much more pleasant than the Phat Phuk, which I find transitions into harsh clipping / brick wall limiting pretty easily when playing agressively with a hot bass, which neuters your note attack somewhat.