AR Sp8 possible breakdown

The SP-8, like most ARC preamplifiers, should automatically go into mute when it is powered up. After the brief warmup, you should here the relay softly click, and then it will unmute. It sounds like your unit has an issue.

Cleeds is correct. All ARC units are in mute until the tubes warm up. When the unit wakes up there should not be any "bang" and both channels should wake up together. You need to contact ARC and find out where your local repair shop is. 

With a preamp or any electronics this old, the first thing to attend to is getting the filter capacitors in the power supplies replaced. This should be done before looking at any other problems, as old filter capacitors can easily damage the power transformer, which is the most expensive part in the preamp other than the chassis, and certainly rare.
Get that done first! Then see if the preamp even has a problem after that.

Thanks for your help. These capacicitors have been recently checked and are in their correct values, also in case that is the reason for the problem; Wouldn’t it be present on both channels?