Well, I have experience with the RG80ES guitar head (which I got royally hosed on in an eBay sale attempt last month). Mine was also an '87 and looked almost exactly like your 500ES. The 80/100ES are metalhead favorites for the gain channel but Randall knew how to make a great-sounding solid-state clean channel too. I bet your amp is a monster - my "80-watt" 80ES was one of my loudest and tightest amps, particularly when using an outboard preamp through the return. It's been my experience that "correctly"-tuned SS guitar heads like that Randall can also make great bass amps too, and with 500 watts... yeah. VERY nice score. I think you'll also find this amp is built INCREDIBLY well / to last the rest of your life if you open it up.
Check to see if any of the pots, particularly the treble pots, pull out. I'm mainly curious... this was an unlabeled feature on the 80/100ES to increase the dirt on the red channel about threefold. (Pulling out the hidden angry pots also, unfortunately, decreases actual channel volume.)