Yeah he offers different options. Mine is the 4550 Deluxe, which is all of his options. It has a PPIMV master volume that is two volumes in series that is foot switchable. So basically you can set the first master higher and the second lower and foot switch between them for a volume boost, or leave it off and have just one master.
It is very much a hot rodded plexi voicing, so it's plex no matter what you do, but the Deluxe allows you to blend channels one and two as much or as little as you want without jumpering them, There is also 3 pots which pull out to change voicing. The mids pot pulls out and is a vintage/hot switch to give a bit more break up or urgency in the amp. It just makes it more aggressive. The treble pot pulls out to accentuate the upper mids, highs, and harmonics, and the volume 1 pot pulls out as a bright switch. This has a brighter effect at lower seetings on the volume one knob, but basically becomes unnoticeable at volume 1 on ten where it is naturally it's brightest.
Then there is the plus mode. you plug into the top right jack and pull out the volume 2 pot and that puts both channels active and in series. Even more gain and somewhat of a change in voicing. Tons of gain. Honestly, I never use this mode. The regular mode is plenty! Whether in plexi mode, which is where I stay, or plus mode you can always blend in any or all of channels 1 and 2 as you want. Usually I am just in plexi mode with channel one at 8 or 10 and channel 2 at 0 to 3. Channel 2 in plexi mode just sort of darkens the amp a little and thickens up the lows if you want. I hope that helps!