still have to turn the unit on, which then powers all the tubes . is my guess
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Best guess (from looking at a picture of the inside) is that all the tubes remain in play. The only thing that bypass switch bypasses is the selector and volume control. At best, only one tube is taken out of circuit. However you just can't remove tubes. This will start to raise the voltages on the other tubes.
It is designed to run with all those tubes, so that's how it should be run.
Something has to supply the gain, Something has to drive the 300B, and the 300B drives the 805.
Preamps typically use the little tubes that produce little heat and cost. If the unit works without the preamp tubes (my guess is it will not)… it will probably not reduce the heat by any appreciable amount. So, if you just want to use as an amp, I would do that. The little tubes last longer and typically cost little.