The power amps role as i understand it is to increase current to in effect drive the speaker ...
The power amplifier stage does what the name says: amplifies power.
You don't think of output tubes as having voltage gain, but they do. It's just often around 10, plus or minus. With a 6L6, you apply maybe 30 volts or more of grid input signal, and have 300v or so of output voltage swing on the plate.
But there's also a current swing. In an ideal transformer, power in = power out, so the input power (plate voltage swing * plate current swing) equals the output power on the secondary. The transformer also allows transformation of impedance (and at the same time, of voltage and current), so while you have large voltage variation with small current variation and high impedance on the primary, you get low voltage variation with high current variation and low impedance on the secondary.
... the phase inverters role appears to be to obviously to invert but how does the amount of current the pi tube is pulling effect its role in the power amp. Does it allow it to hit the grids of the power tubes with plenty in reserve ...
In a typical guitar amp, there is no current drawn by the output tube grids unless you drive the grid until the instantaneous voltage is more than 0v. When you do that, you already have all the clean output power, some distortion, and are on your way to severe distortion.
The primary job of the phase inverter is to provide 2 output signals of opposite polarity to drive a push-pull output stage. It may also have voltage gain in addition to its primary job. That the tube used could also deliver some amount of current along with its output voltage is a feature not utilized in a typical phase inverter. The place that feature is used is the driver for a push-pull class AB2 or class B2 output stage. These often use an output tube as the driver (6V6, 6L6, EL34 are common) and a transformer to couple the driver to the output stage grids. These particular classes of operation assume current is delivered to the output stage grids along with the voltage, and current * voltage is power.
The confusion is that we start with a complete circuit, and then try to learn how it works, rather than the old textbook way of learning basics and then figuring out how to make a circuit. If you read old textbooks from the 40's and 50's, the typical triode to use as an example of a voltage amplifier is a 6J5 or similar (something not unlike a 12AU7, with a mu of 20 or less). This tube has what would have been considered a medium mu, moderately high transconductance and moderate-low internal plate resistance.
The fact of low internal resistance means that if you apply a voltage across the plate and cathode, more current flows than if the plate resistance weer higher. The material used to make the plate (and its size, etc) means it can handle a certain amount of power (again, voltage * current) without overheating.
So mu is low by modern standards, internal resistance is low, and given the power rating the tube could handle a fair amount of current flow. It was found that voltage amplification could only approach the value of mu, and that it takes increasingly higher load resistance to get there. But in order to increase load resistance without other results that wreck voltage gain, you have to keep raising the power supply voltage. Ultimately, it was found that a load resistance much greater than 4-5 times rp (internal plate resistance) results in very little additional gain.
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