The Line 6 modeling pedals often suffer from a volume drop when activated. Specifically the MM-4 and DL-4 suffer from this. The unity gain mod increases the volume just enough to even out the volume to the level when it is bypassed. The sonic overhaul mod upgrades the stock input/output chips to high quality Burr Brown chips, adds unity gain, and removes the unnecessary audio signal RF filtering (only present on the early models). A switch upgrade is also available- this replaces the tiny PCB mounted switches with soft touch stompswitches for a more dependable unit.