Originally Posted by sprung2
What you're describing is not destructive but constructive interference. Which is usually desirable. Modern receiver implementations of multi-channel stereo have no phase cancellation issues with monopole rears.
We were both wrong. It's actually destructive and
constructive (dips/peaks). He said he was using them for stereo sound, not multi-channel stereo sound, so yes, there will be phase issues. Regardless, playing a stereo source in multi-channel stereo destroys the fidelity (albeit less so than having phase issues).