If its a Standard Pioneer receiver, more than likely, when "A+B" is engaged, the receiver is either running in series OR there is a 4 Ohm resistor in there to maintain impedence. Is there a switch on the back of the receiver for 4 or 8 ohm operation?

If its an Elite series receiver, the amp MIGHT be good enough to do what you ask, but would never recommend or do this. A receiver just does not have the current to run a high power sub.

What receiver and exactly what sub are you trying to run? If its some flea market special and you really dont care about sound quality and longevity, you could experiment a bit.

Otherwise a Properly build cabinet, and a dedicated power amp with crossover is the way to go.