But could a bad MIDI cable really cause the Studio 5lx to crash & burn?
No. Absolutely not.
Anybody have any tips that might give me the confidence to try plugging it in and turning it on again?
Hoping for a different result? That's insanity. Don't.
Could it have just been a fuse?
Fuses don't do that.
The spark & pop seemed to come from somewhere near the front-panel A/B switches.
It likely seemed that way because the front was all you could see. It's barely possible the power switch failed somehow, but…
…far more likely it came from the power supply which is in the back by the AC inlet. Get out your trusty Phillips screwdriver - the correct one that actually fits because you'll quickly trash the screw heads otherwise, take the top off and take a look. The PSU is right where the AC cord connects.
robespierre has a good idea there. The EMI can and does just decide to retire sometimes. You know, using old electronic hardware is a lot like driving a vintage car. If you're afraid or unwilling to get your hands dirty once in a while, vintage is not so much for you.
That said, >90% of Studio 5 fails are caused by the power supply. That's where the heavy lifting gets done. If not the EMI module, it will be a filter cap or ehh…maybe a diode. What ever it is, if it made noise and let the smoke out, you can probably see the damaged part burned or at least discolored. It's also likely that the fuse will be blown, having reacted faster than you did as it's supposed to.
That said, if it's proven to be the PSU, any decent tech should be able to fix it easily if you can't or won't.
Barring that, anybody want one for parts?
Personally, I have three - one working, one for backup and "one for parts". That's another good practice when using hard to get or unobtainable vintage stuff.