So I'd assume that you are running the sub off of the 7100 by using the RCA's out of the back right? You might have a level adjust that will allow you to turn that up or down. Also gains all the way up on an amp are a big NO NO!! If you have to do that then you have a problem.
If you want to be able to "turn down" the volume and try and level out the system you could buy one of these. VARIABLE 4-CHANNEL HI-QUALITY AFTERMARKET DECK TO OEM SYSTEM ADAPTORS N-684V
From this web page.
http://www.autosound2000.com/adaptor_products.htm
This will allow you to level match your system.
Or you could just buy a decent 4 ch amp and some new speakers or at the very least new speakers and just run new wires from you 7100 direct to the speakers and power them off the head unit and bypass the factory amp all together.
If you want to be able to "turn down" the volume and try and level out the system you could buy one of these. VARIABLE 4-CHANNEL HI-QUALITY AFTERMARKET DECK TO OEM SYSTEM ADAPTORS N-684V
From this web page.
http://www.autosound2000.com/adaptor_products.htm
This will allow you to level match your system.
Or you could just buy a decent 4 ch amp and some new speakers or at the very least new speakers and just run new wires from you 7100 direct to the speakers and power them off the head unit and bypass the factory amp all together.