Most of the time, yes Myles uses the amp with the channels jumpered together. He sets it with the treble channel on about 4 and the bass channel on about 2 or 3. My amps both had a master volume pot added after the preamp section, similar to the way Marshall did it on the master volume amps. It's silly really because the 4 input top sounds terrible if used master volume style. I've left them like that however because it's sometimes handy to use it to "thin out" the sound a little, you just turn it down a tiny bit and it tends to roll of a small amount of bottom end. We use the amps into a Power brake most of the time anyway to help control overall levels and how hard it hits the cabinet.
Jumpering the channels together adds the gain of each channel together. It's not really like the master volume amps where the channels are cascaded and the gain from one stage to the other is multiplied, it's just not that kind of preamp distortion. It just gives you a bit more control with whatever guitar you happen to be using.