If the power is needed for the output would running it at a lower power only reduce the output or would it have other effects.
I see the output varies as to the resistance of the speaker attached and as I haven't got the speaker yet I could go for a lower resistance to increase output.
Also I don't really need a big output anyway.
The design I found had the power supply preamp and power stage separate which made it easy to change the design and put in a different preamp.
It reduces the output (a LOT!), but may also require the power amplifier redesigning to take account of it.
I see the output varies as to the resistance of the speaker attached and as I haven't got the speaker yet I could go for a lower resistance to increase output.
Usually the preamp is fed from the main supply via big resistors and a pair of zener diodes (crude regulators), but these also isolate the preamp supply from the power amp supply - which is also important.