The Route 66 is essentially two famous circuits modified for modern players:
Overdrive (The "Route 808" side): Historically based on the Ibanez TS808 Tube Screamer, featuring the iconic JRC4558 op-amp. Visual Sound modified this circuit to include significantly more volume and a "Bass Boost" to fix the low-end loss common in original Tube Screamers.
Compressor (The "Comp 66" side): Based on the classic Ross/Dyna Comp circuit but with an added Tone control. This allows you to add back the "jangle" and high-end clarity that compression often squashes.