The power tubes are a pair which form the two halves of a push-pull circuit. This is why matching gets so much attention. I don't have a schematic for your amp, but usually the preamp tubes closest to the input jacks are the first tubes to work on the signal from your guitar. The preamp tube nearest to the power tubes is probably the phase inverter, which splits the preamp signal and sends it to the two power tubes. Any tubes in between are for the tone stack, reverb drive, tremolo oscillator, etc.

Take a look here and here for some more info.