Overall the project was only a days work, low cost mostly using parts I had on hand. The result works great, with amazing sound and power for such a tiny amplifier. My intended use is to provide portable well distributed sound in a conference room and it will work perfectly. The amplifier, power brick, all 4 speakers, 200ft speaker cable and 50ft extension cord all fit within a 16x10x5 box. The issues with speaker pop and bluetooth sounds can be avoided using the volume knob, turning it down when powering and connecting, turning the source to max, then using the volume knob to adjust.
There a a number of things I might do in the future.
Antenna. I might look into putting a better antenna on it to increase the range.
External input. I'd like to have a line input so that it could be used without bluetooth if possible. It wouldn't be hard given the volume is inserted at a line level, would just need to switch over it's input and AC couple the line.
Auto muting/anti-pop. In thinking through how to deal with some of the issues, I realized I could add an output relay so that the speakers are disconnected before the power goes off, and connected after a delay upon power up. This circuit wouldn't be difficult and would eliminate the pop and connecting tones(if connecting right away).
Volume pot. I think the slight hiss at loud volume would reduce if I changed the pot out for a 1k device.
Power switch. In some use cases this might be handy, especially if I make a battery powered version.
In testing, I've found it quite handy to have this little amplifier and have been thinking about building a bracket that would hold the amplifier, 2 of the speakers and a battery pack all together. The result would smaller than a shoe box, and yet much more powerful than most on the market. Given that it works fine on 12V, I would probably use a 4S hobby battery pack which could easily have a very long run time given how little power this uses.