Hum in one speaker?

Sounds like you had some schmutz on your connection. One of the symptoms will be that you will pick up more noise (including hum), and another symptom is that it will be sensitive to vibration (tapping on it makes it cut in and out).

The schmutz causes a high impedance connection, which still can couple in the signal, but the high impedance connection becomes more susceptible to pickup of stray magnetic fields. It's like you created an antenna.

The high impedance connection of the dirt also creates a variable resistor, which changes value under the pressure of vibration. The vibration causes changes in connection impedance and often causes a crackling noise as it changes value. Vibration of the surface the preamp is sitting on becomes modulated into the signal through this variable connection. It is one form of what is commonly called "microphonic noise".

Clean your connectors. I use rubbing alcohol.

Sometimes cables fresh from the factory are the worst for schmutz. There can be like this oil stuff on the connector, that is a residue from the manufacturing process.

And what is worst is when you have the oil get from the jack into the socket, because it is harder to clean then. The way to clean that out is to first clean the jack with alcohol, then plug the jack in, jack it in and out, and then clean the jack again. Repeat until you get a solid connection every time you plug in. Your using the clean jack to wipe out the socket.

A totally different topic... if you crank an amp up and the hum gets relatively louder than the music, it can be a sign of having an electrolytic power cap in the power supply going bad. What happens is that the cap is weak, and at high current draw of the amp, the weakness shows up as being significant, but at low volume the cap is able to keep up with the current demand. What was a hefty 4700 uF cap has degraded to a 500 uF cap, and it is not able to smooth out the 60 Hz ripple of the AC power supply when load current demand gets high. There isn't much you can do about a problem like that, the cap will progressively degrade over months or years, and eventually fail altogether. The cap has to be replaced.

If the hum is coming in through the signal path, the relative volume of hum to music volume will remain the same as you crank up the amp.

I think your problem is schmutz and not a cap. I mentioned the common cap problem because the symptoms are similar. I wanted other people who are reading this to know how to tell power supply hum from pickup hum.

When the noise is coming from pickup in the signal path, the signal to noise ration remains the same, because you are faithfully amplifying both of them. When the hum is a droopy cap in the amp power supply, the signal to noise ration will be worse when you crank it up. It might even start "motor boating". Also, a power supply cap that is on it's way out gives off an acidic smell, like lemons.