If you have time and want to learn more about Linux you might want to set up and play with a souce distribution (i.e. Gentoo) at home or on a spare VM / PC. The benefit of this is that you can build a system from the ground up and this will teach you how the components hang together.
Gentoo Linux Documentation -- Gentoo Handbook
While you can't deploy cPanel on top of Gentoo, and it doesn't use RPM (the packaging system increasingly used by cPanel) learning things such as how to compile your own Kernel can be very relevant to security. If you're not using cloudlinux on your cPanel box(es) you can compile your own kernel with grsecurity to get some good protections.
From a familiarisation perspective, it can also be a good idea to move to linux as your everyday work os, this forces you to fix little issues on a regular basis which will help you learn no end.
If you're just starting out on the security side of things, reading about the differences between the various ways of serving php and understanding exactly how junk mail can be sent from a compromised 1) individual email account 2) hosting account (i.e via a php script or cgi) or 3) the server itself at root level are good places to branch out from.
The best grounding is undoubtedly a good comp sci course for a whole bunch of reasons, but increasingly materials are available online including exercises on breaking out of chroots etc which you can have a go at once you have the OS fundementals down.
