If I remember correctly, the power amp is what actually powers your speakers and send the sound through them.

So, if you don't use it......

You're pretty much going to be silent

How do you say "I'm okay" to an answering machine?

boosts line level from the preamp section to speaker level.
Jackson Rhoads EX
Jackson RR3

ENGL E530 preamp
Peavey Classic 120 power amp
Randall Cab