Don't know if you got your question answered or not having had a quick read through. I'd say power amp have been used historically to amplify vocals, keyboards, drums, brass etc. Guitars and Bass amplifiers tend to have preamps to colour the sound with EQ and various other effects and functions. A power amp is a completely clean sound with no colouration. Bass players have started using power amp setups now that there are a myriad of preamp pedals and multi effects units on the market which create the tone they want. A standard bass amp has a preamp and power amp in one unit.