| It's not easy to follow that schematic, the resolution is not very good. Also this appears to be like a lot of modern amps, the layout is complicated and difficult to follow through the schematic the way it's drawn and I don't have time to really go through it thoroughly.
But it appears that the input hits V1B for the initial gain stage/buffering. Then V1a may be switched in or bypassed, and the signal apparently can also bypass that combination with V2B. But V2A is always in use it appears.
Now, about V3A and V3B. In case you may not realize, 12AX7 tubes are dual triodes, so one triode section is labelled A and the other section B. They can function completely independently. Here, both sections are fed with the signal from V2A. Now, when Vintage is switched in, then V3B is engaged and its signal goes through the tone and volume controls for Vintage. When Hot is engaged, the other section of V3( V3A) is in use. These triodes are both set up as cathode followers. So one or the other section of V3 is always in use.
V4A appears to drive the effects send, and its a cathode follower as well. V4B is the effects return buffer.
V5 is the PI.
You might want to try a 5751 in V2 for lowering gain a little on the clean. It's better than using a 12AU7 because the 5751's characteristics more closely matches the 12AX7, only with 30% less gain. | 
