I have owned the ARC Classic 60 since new.
Agree on the above comments in replacing the Caps. I would not send this to ARC since it will be very expensive. I just had mine rebuilt in the last few Month’s by Scott Frankland Associates who is very familiar with this Amplifier. The 4 Large Caps with Red Tops in the center of the Amp are called Filter Caps and Scott was able to source new ones that fit so these were changed.
There is also the problem of Tube Auto Bias on this Amplifier which does not work well. Every time you have to change the Tubes you need a Technician to Bias this Amplifier for you. If you do not have a Tech close by then it is a very large hassle with this Amplifier. Scott modded mine with test probe locations and adjustable pots so I can check my own Bias with a Meter since I was tired of lugging this Amp around for someone to Bias it. It was a brilliant solution.
If you wish to get by for now with your Cable connection problem order these;
https://www.partsconnexion.com/VAMPIRE-55446.html
They fit perfectly and they can be crimped or soldered. Two will also fit easily on each of the + or - Channel Screws. I do not recommend the Adapters mentioned above.
Do not use Bare Wire.