Powering up a SET amp after 6 months in storage

Six months is hardly a long time to be idle. I would just remove and reinsert the tubes (or better, clean the tube pins and the socket) to assure good contact and then power the amp up.

The concern with long-term storage has mostly to do with electrolytic caps that have begun to dry out. These can be brought back to life, but it is best done by slowly powering the caps up. But, this is a concern after years of storage.

Also, if your amp utilizes tube rectification, power naturally comes up a bit more slowly than with solid state rectification, so capacitors are given less of a sudden jolt.

Don't worry about it. Six months is nothing. Turn it on and enjoy.

To bring up slowly you need a variac which is basically a giant variable resistor. You start out at 10% of rated voltage and bring it up in 10% increments every 10 minutes or so. I believe this is primarily for the capacitors, but it is good for tubes too. Some tube electronics have soft start circuitry for the tubes just to avoid shocking them on every turn-on. Six months of storage is borderline for doing this, if you know a good tech who would do it for low cost it would be worth it, otherwise it's a judgement call.