Using stereo tools. Or not?

I'd recommend a mono-compatible plug-in that only affects the side signal (especially if used on the master). is free and you can lock in "fix Mid level" so it doesn't impact the mono channel

- always check your sides and your mono. 

Here's a quick demo. The two sends are 1/2 and 3/4. 1/2 is just a regular ole send, 3/4 is the mirror image, both are the same reverb it's just the send used, that dictates whether it is mirrored or not. I'm sort of playing with both of them so you can get an idea (I begin by adding 1/2, non-mirrored). Hopefully you can see/hear that you get more creative choice with placement in whatever space you are trying to create and IMHO helps with widening in general.