Hi there,
If you love your Plexi, you will probably be pleased with the XTC, i prefer it over the SL+, nice long sustain and very good crunch depending how you set them, i'm even thinking about a second XTC, one for lead and the other for rhythm !!!
I find that they complement each other very well. Those are the 2 mods that i use most of the time.
For the Blackface it stays super clean unless you set the gain very high, much like a Twin.
This module is my prefered clean, but i have to say that i'm very pleased with the Plexi clean when the gain is set low.
Hope this helps ...