r/line6

So I have this 100 watt amp that says "2x50 watts stereo" right underneath the name plate, and one of the speakers has recently blown. It's still loud enough to gig and play live shows with no PA, so has the amp been delivering 100 watts to one speaker or only 50? Have I actually been playing gigs with a 50 watt 1x12? Sorry if this is a silly question but I have no idea how amp / speaker wattages and wiring work.

I have a line 6 spider IV 30w combo amp and i want to sell it. However i cannot decide the price. What do you think?