Why did my amp just crash and burn ?

Sorry,  thought you new cables were interconnects not speaker wire, should have read more thoroughly 

Your a lucky man, all is well. Be sure to match fuse exact, after this post I’m going to turn off my amp when switching interconnects or any other wire from now on OP you taught me a lesson.  I just ordered 10 fast blow ceramic fused from Mouser .62 cents each, if you need a place to source correct fuses. Conrad Johnson charges $7.95 per fuse at .62 might as well get a life time supply.

Amp took out a fuse last week on the output tube protection circuit. Had the old xf2 tubes tested there all good. Conrad Johnson said that is normal and keep using the old Mullard’s until it takes out fuses repeatedly. The tube did not red plate. That’s very confusing to me for him to say that, got to ask while I’m here anyone heard of this? 
 

I’m sure you new interconnects were not faulty but you could check with a volts meter to make sure signal wire is not crossed with return. If there was a problem or defect with interconnect it could caused amp to shut down.
 

I would think there would be more than one fuse for the amp other than just the one on the IEC inlet. Maybe one on each rail. I don’t know your amp so may not be the case. Good luck finding the problem.

I’ve been in your shoes, I crossed speaker cables on a McCormack rev a DNA 1and it blew the fuses on the rail. Not a good feeling, luckily got it up and running. Hang in there you will have it playing music again soon one way or another, YBA makes very nice musical gear.

itsjustme is correct turn it off. I’m still guilty of changing out interconnects as long as I change the source.  Most of my amps are tube and I don’t want to wait to turn it back on. I’ve learned to turn amp off when playing with speaker wires I double, triple even quadruple check speaker wire before turning amp back on. I had to learn the hard way when I blew up the McCormack. 

You may have grounded positive speaker wire to chassis while handling speaker cables. Like someone said open up the amp look for more fuses. Look for burnt resistor, black marks on the board, look at power supply electrolytic caps usually the tops where the cross is are swollen, even swollen film caps. Look at it as a learning experience what not to do, that’s what I had to do, Good luck.