"How do you determine the current output of a tube amp as manufacturers (tube or SS) do not spec those in their data sheet"
The current delivered by an amplifier is a function of the load that the amplifier "sees" and is not a constant value. Some brands include peak to peak amp, etc., in their literature but its purely a marketing gimmick, in my opinion. The rule of thumb for an amplifier with a good current delivery, which is basically a function of a good power supply, is doubling the watts as the load is halved, 200wpc @ 8 ohms, 400 wpc @ 4 ohms, and down the line.