Next there is a comment that adding more tubes will not help unless the voltage rail is increased. Thats not right - 3 pairs of tubes will deliver 1/2 again as much current into the transformer as 2 pairs - giving you 1/2 again as much power with the SAME voltage.
Here is where my understanding fails. For a given load resistance, in order to deliver more current more voltage must appear. If we parallel tubes we deliver more current to the windings but the voltage on the output winding is set by the input voltage and the winding ratio so we can't deliver more voltage to the load. The only way I can figure is the input current is increased transforming to increased current to the load which causes a greater voltage to be dropped across the load which is reflected back to the input winding causing plate voltages to skyrocket far past B+, but of course this is impossible since the power supply would prevent this by sinking the excess. This is an infinite loop in my head that I would love to see broken...
Don
BTW Thanks Ian for taking the time.