What he's talking about is limiting the bass early in the gain stages. There are five gain stages and the tone stacks are at the end, so the bass is already being pushed into distortion and "muddying" the tone. To tame the bass in the early stages, you need to drop the Cathode bypass caps, (from 1uf to .68uf or .47uf) and/or drop the coupling caps, from .02uf to .002uf, .0047uf, or .01uf).
If you only introduce the low end at the last couple of stages, it sounds like you are running your low signal through a clean channel, and the mids and highs through the distorted channel. It gives the effect of smoothing out the signal, making it sound tighter.
But it DEFINITELY changes the character of the amp, making it not really a Recto anymore. Randall designed it to be bass heavy and a bit scooped, and with that bass being distorted so early, chugga chugga's sound quite a bit bigger than other amps that may have a prettier sine wave.