Aragon 24k humming on left channel

My system is based on this Aragon 24k feeding into an Aragon 4004 Power. Lately is has developed a ground noise or hum on the left channel. Very noticeable with the phono input, much less with all the line ones but still there. It also increases when you put it in the “mute” position. I have heard that it could be caused by a capacitor or capacitors gone bad at the pre side. I also heard that it could come from the external power supply. Music quality is pristine as it has always been for the last 20 years that I had it, but I would like to take care of this if possible. Any help is highly welcome. 
Likely needs recapped and if I had to guess it’s the amp. Try to isolate which component it is if you can. I think if it’s the amp and if you leave the preamp off the amps going to hum regardless. Both should be easy to work on so hopefully you have a good local tech that can help you or call the new owners of Aragon to see if they can help. Indy Audio.
I also heard that it could come from the external power supply.
If hum only on the left channel, you can rule out the external power supply.
Thanks for your input. @imhififan you are right! @adg101 I tend to agree. Actually you can hear a humming coming right out of the amp (not at the speaker) when you turn the power switch on. For the moment the hum on the left channel is low enough to continue coping with it, but if it gets worse I will have the amp checked. Thanks!
Sybesma's electronics in Holland Michigan used to repair Aragon and Acurus gear. I am not sure if this is still the case, but you contact them and find out.
Update. I am taking both amp and pre to my local tech. Once it’s clear I’ll post the results. Thanks to everybody that gave me tips!