Transistor amps are not very friendly places for the inquisitive. The beauty of old tube amps is that they were built by point-to-point stringing a few components together on turret boards. This makes for untidy assemblies, but they're so easy to modify. I have no idea what's in a Frontman, but I'm pretty sure it's all built on printed circuit boards. A lot harder to do anything with.

Start out small. A one-tube, single-sided Class A output. That's where an old tube radio comes in. Pick one up from a flea market for a few bucks. Find a schematic for it. If you're lucky, there's one in the cabinet. Get some tube amp books from the library, or buy a couple from StewMac. Look up the data sheets for the tube(s). Figure out how to separate the radio section from the audio section and feed your guitar into the amp instead of the radio. Study the biasing circuit, and see what effect different component values have. Look up some tone control circuits and add them to your system. Get a thorough understanding of the one-tube, single-sided Class A output circuit, and then move on to a two-tube, push-pull circuit. See how biasing can change it from Class A to Class A-B. Understand what the difference is, and how that affects the sound. Keep pushing, keep learning.