HELP - Vox AC15C1 Loud Hum

I recently plugged into my Vox AC15C1 and was greeted with a very loud hum. Trying a couple of things, I decided it must be a bad tube and ordered a complete new set of tubes. I replaced the tubes today but I'm still getting that loud hum. Here's what I tried:

Different guitars with different pickups (SC and HB) - no difference;
Different instrument cables - no difference;
Different amp inputs (Normal and Top Boost) - no difference;
Different amp power cables - no difference;
Reverb/Trem footswitch connected and not connected - no difference;
Different electrical outlets - no difference;
A complete set of new tubes (all five were replaced) - no difference;

Hum is not present until Stand-by switch goes to ON;
Hum is not present until Master Volume is turned up;
Once the tubes are warm the hum isn't audible with the Master Volume turned completely down;
Hum increases and decreases in volume with only the Master Volume;
Normal volume and Top Boost volume knobs seems to have little effect;
Hum is present whether an instrument is plugged in or not;
You can barely hear the guitar when it's plugged in.

Tremolo will affect the hum;
The amp is about ten years old with the original stock tubes (until I replaced them today)
The amp worked fine the last time I used it approximately six months ago. Nothing changed since then except the introduction of the hum.
It's a Vox AC15C1 but not hand-wired.
The bright cap mod was never done on the amp.
All stock with the exception of the reverb tank, which was replaced approximately nine years ago.

It sounds like huge grounding issue or - from what I heard on a YouTube video - a bad filter cap.

I guess the new tubes (or one of them) could be bad (purchased from the Tube Store) but what's the possibility?

Any suggestions? Can filter caps be replaced on this amp model?

Outside of changing tubes, anything else will need to go to an amp tech but I like to know what the possible issue might be and if it can be corrected/fixed.

Thanks.

 

It appeared to close to the 60Hz hum but...

I downloaded an Audio Frequency Counter app on my phone and it read 240HZ. I then downloaded an Audio Frequency Generator and set it to 240HZ and it pretty much matched the hum of the amp. Is 240Hz even a thing for amps?

I know I'm just playing around now because outside of changing tubes I really don't know about any of this.

Going to see is my local store can take a look at it tomorrow.