I guess I'm the person to answer this, as nobody else has one yet

so a 12AX7 is what we call a dual-triode, which means it has two stages. The blue (clean) channel of Smoke Signal uses two of them, the red (dirty) channel uses all four. Obviously this means that two of the stages are shared between the two channels.
The channels are voiced very differently, as mentioned above. The red channel is very much a gain channel and the blue channel is very much a clean channel. The blue can drive a little when cranked, and the red can clean up a little when the gain is dialed back, but they are clearly a clean channel and drive channel.