As above, what are the symptoms of a faulty amplifier?

It's symptoms are when things doesn't sound or act like they should.

What do you mean by Faulty anyway?

If you can list down what's WRONG with your amp, then we can help out.

Erm, I'm not sure whether my amp's faulty or my guitar is which was why I wanted to find out symptoms of a faulty amp first. But anyway, when I add a little distortion, there's like a very low hum during each note. These hums only appear when I play different strings.

sometime cap ayam pickup will produce humming sound when you increase your gain.. anyway try your amp with another guitar and try plug in your guitar to another amplifier.. from that maybe you will know which one was faulty..

I see, must be my pickups. Will update you guys asap.

Not really. Many branded pups will produce unwanted noise too, especially singlecoils.

Is your gain maxed out?