Wholesale, assuming the VTM and KT-120s are compatable, definitely go the KT-120 route. Check with ARC to confirm compatibility. I dropped them into my VS-115. Major improvement across the board.
Now as far as the possible need for an electronic repair --if a bias resisiter blew, the part costs about a buck. I even bought a few back-up sets just in case. If you know how to do simple repairs, you might be able to eyeball the bias resisters and see if one looks blown. In my case, neither I nor my ARC tech could visually see the faulted resister. The tech used a tester and found the fault in about 5 minutes -- most of the time was spent cracking the amp open.
I would be remiss if I didn't warn you about a serious risk if you crack open the amp. Our amps store potentially lethal amounts of high voltage. If you don't know what you're doing re testing and replacing a defective resister or any other part, DON'T DO IT!
Call Calvin at ARC. He can walk you through a few diagniostic procedures that might help to localize the fault. If as I suspect the problem is a blown bias resister, he can either walk you through the repair or refer you to a local ARC authorized repair tech.
In my case, there's an ARC authorized tech whose shop is near my house. Rather than schlep a 70 lb amp around, he came to my house for a small fee and replaced the faulty resister in about 45 minutes. Afterwards, we shot the breeze and listened to some 70s music for a while. Ya gotta luv ARC.
Bottom line -- don't be cheap or a hero. Call ARC and get some solid advice. Call someone who knows what they're doing. If it's just a bias resister, the fix is a snap!!
Please let us know how you make out.