It really depends on what kind of sound you are going for. I like the sound of a tube head so I use a 6 pro.
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Power Amp: Mosfet
Actually power amp: tube / mosfet hybrid.
Both the 3PRO and 6PRO power amps have tubes for all the gain, with the mosfets acting almost like an output transformer... no gain, just "impedance matching" between the tube plates and the speaker.