What is the purposes of using a power amp with your regular amp? Is it to avoid loss of volume after long periods of time playing? Is it just to boost a signal? Is a cheap power amp better than no power amp? Etc...

If your using a guitar amp you don't need a power amp as it already has one. Now if you have a preamp you will need one. I prefer tube amps myself, but their more expensive of course. The tone is also better.