What did you change?

For some reason, the Mode switch on my amp had been set to L+R Sum, which is
where you're meant to set the amp when driving a Mono Bass or Center channel. I
switched it to Stereo, which widened my soundstage by 200% and created real
stereo definition. There's another switch, next to the crossover dial which
reads "full", "high" and "low. They had it set to "high" and I've been doing
listening tests with it set both to "high" and to "full".