AR Sp8 possible breakdown

Your unit needs to go to ARC for immediate service before you have a serious problem.

thank you very much for your help. And if I change these capacitors, the next problem is which ones to put so that we don't lose the essence of the original sound.

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?

Correct value isn't so much the issue as whether or not the caps are drawing power. Usually you would expect power supply issues to manifest in both channels, but when the bypass isn't correct that isn't a given. So if the caps are original get them replaced- if the power transformer dies you'll really be regretting it and they smell terrible when they go.

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?

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.

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. 

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.