Fix Aesthetix Io volume control?

Get a new quality volume control (potentiometer) installed if it's not a bad solder joint.
The volume control even a quality one is the single worst component in an audio system.
It consists of a very thin spring loaded metallic (usually brass) wiper than has a very light pressure, point contact on a resistive track, either made from carbon or conductive plastic.
These are dissimilar materials as well, which is bad enough, and with a touch contact that the line level signal has to pass through. If the same contact was one of your RCA's or XRL's plugs you would throw it in the bin and get a new one. The same should be done with volume controls once they used too much.

They make "reasonable" contact when new but start to degrade from the first time they get rotated back and forth as the point contact starts to scratch and groove away at the soft resistive track, and wear starts from day 1 when new when it first gets rotated.

Cheers George

The Aesthetix Io does not come with the average volume pot.It has two "mechanically switched discrete resistor volume control with 46 positions using Roederstein resistors". It is probably too complex for me to fix - I was just wondering if anyone had done it.

Yes this is more complex for you to fix.
If there is no sound from it.
If it's switched resistor (series/shunt) volume pot then it could be either the wiper series resistor is not contacting or you have a dry joint on the input output or ground wires.

If it a switched resistor (ladder network) then you could have one resistor that has gone open or one of the resistors solder joint is open, also the input output or ground wires.

Cheers George

O_holter, about 7 years ago I had a crackling noise in one
channel of my Io as I rotated the volume control. My thought
was that something had gone wrong with the volume control. I
sent it back to Aesthetix to resolve and when Glenn got it
on the bench, it turned out to be a loose socket for the
6sn7 tube. He replaced the tube socket and I haven't had a
problem since.

Yes by all means O_holter, I thought you nailed the problem down to the volume control yourself, but yes if you are not sure, then Rushton has a good point to make.

What you should do is lift the lid to swap tubes left to right to see if anything changes, or very slightly wobble them and listen to see if you get noises or music coming back.

Cheers George