Any amp with multiple inputs acts to some extent as an audio mixer. A stronger guitar signal will foul up a weaker signal by forcing its signal into the weaker pickups. If one of the guitars is played through a pedal, the pedal can seriously ruin the other guitar's signal. So at the very least, each input jack should connect to a common preamp through a resistor. Common values are 47k, 68k, and 100k.
Higher values give better isolation, but also reduce highs somewhat, and reduce input volume.
This, of course, means more tube stages are needed. The tradeoffs here are component count and circuit complexity versus sound quality when multiple inputs are utilized. Of course, if you only use one input jack, this is all moot. Integrated Amplifiers Jasniewski suggests sticking with PA amps, and in general I tend to agree. The problem here is that vinyl phonograph records were recorded with special tone compensation and the playback circuits have to undo this.