Keeley positioned the Super Phat Mod (KPHAT-U) as the production realisation of their long-running Phat Mod modification — a circuit built around JFET devices selected to emulate tube-amp behaviour. The design carried forward tube-like amp stages, tube amp tone stacks, and a variety of tone-shaping circuits drawn from the original modified platform, which had remained in use on players' boards for nearly 15 years before this unit was produced. The specific JFET device ultimately chosen for the Super Phat Mod was a Toshiba component identified by Craighton Hale as capturing the harmonic content and grain the earlier mod was known for.

The enclosure was finished in metal-flake clear-sparkle over gold with a royal purple colour scheme, and the name badge used a Blackface-style logo. The circuit includes a micro toggle switch that selects the Phat mode — a bass-response voicing originally developed for the Keeley BD-2 mod to give players of single-coil instruments a low-end push without excess muddiness. True-bypass switching is fitted. The design is described as full-range and highly responsive to pick attack and guitar volume control, a characteristic the phrase "transparent overdrive" was coined to describe. The Super Phat Mod shares lineage with the Luna Overdrive and the subsequent Katana Blue.

Owners needing setup adjustments, internal inspection, or switch servicing should consult a qualified guitar technician familiar with effects unit repair.