If you want to get 60W out of the amp then be sure to use appropriate gauge wire for the power, ground, and output to speaker wires. At 60W you'll want wire that can handle up to about 4 amps to be safe. But if you only plan to use it with the battery or a 9V-12V power supply then wire gauge shouldn't be much of a concern. If you choose to use a power supply then make sure it's a good quality low noise power supply. If it's a switching power supply then you'll likely want to add additional filtering caps. If you don't then you might get a lot of white noise, but it will still work. Also if you want the circuit to be protected from connecting a power supply of the wrong polarity then you could add a Schottky diode in series with the power rail directly off the DC input: amzn.to/4cbHAFG
Also (again), you could build different preamps and put more than 1 in the case. Then you could select what kind of amp you're playing by turning a switch. Or am I over simplifying this?
You're correct, that's a great idea! If you wanted to switch between two different preamps then this is the switch you'll want to get: amzn.to/3PdY2f1 It would be wired similar to a footswitch in a guitar pedal, minus the LED.
I like to consider the 100% efficiency case, V=IR. R=8 ohms, if the current rating for your boost converter is 1A, that gives you 8 volts max before the power supply starts to droop.
Your assumption that the 8 ohm speaker is connected directly to the regulator's output is incorrect. From the regulator's perspective the power amp is the load. If the power amp is given 9V then it will output up to 5W with an 8 ohm speaker according to the charts I showed. Since the amp is roughly 90% efficient then its actual power draw will be around 5.6W. The preamp barely uses any power, only around 0.2W max. Peak power use of the entire circuit is less than 6W, and that's at max volume. The boost regulator is capable of 1A output (or 10W max), so if it's set to 9V output then it can produce up to 9W which is considerably higher than 6W. There's plenty of headroom.
