[poty]

I had initially thought that too, but where would the high voltage AC come from?  The only AC present is signal level, from the amplifier stage. 

Across the tube there is 175V. If a shortage happens the voltage starts to charge the output capacitor through the preamp input resistance. This impulse can damage anything.

When I verify that there is no DC on the output, do I touch the probe to the outside or inside of the RCA jack? Or both?

Outside will be OK. Be very carefull, because without loading there could be some potential.

[Biff]

Just checked - zero. Set it on the lowest DC setting, nothing. So I think we're okay. Again, many thanks to all who responded.