Having A & B speaker terminals does not make it a four channel amp. Many used as such suffer when trying to use A&B together, as they don't actually have four discrete amps inside, A&B terminals are more about using in two different rooms and not necessarily together....but depends on the impedance load of your speakers, so good to know what that is.
ps from what I can see there are no pre-outs to add a separate amp with, altho since you have a separate volume control you might be able to use the tape outs for such (tape out wouldn't be controlled by the volume knob on the Integra).