As far as tubes... I test 'em, replace weak ones, and if it doesn't make a noticeable difference, I put the weak one back...
I'd have a replacement set of tubes, but I wouldn't necessarily CHANGE any... Although I HAVE changed god tubes when they weren't the original brand and I had one that was.
17JK8 will cost you $1.80 at Antique Electronic Supply. If you're desperate, a 12DT8 would probably work, or a 17EW8... but it is an oddball tube, with two unequal triodes. And 50EH5 costs half of what a 50C5 costs...
If the cord clamp make you nervous, wrap the cord with Nomex or similar tough insulating material where it goes through it.