Forgot to mention BX digital’s ‘Stereo maker’ plugin as well, which is specifically for adding width to mono sounds, works better on some sounds than others and takes a bit of experimenting to get things right, but again in mono it causes no issues.

Not pimping the BX stuff, I just know it works without screwing things up when going through mono systems.

There are also some from PSP that work well. Most of the good ones have correlation meters on them too.