Audible Illusions hiss

The preamp is probably 15 years old but has had very minimum use for much of that time. The tubes were purchased last year from The Tube Store and have less than 100 hours on them.

Minimal use is actually a bad thing for capacitors. Those units also have a regulated filament supply and its possible one of the transistors or the IC has gotten noisy. That would show up in both the phono stage and the high level stage but might be worse in phono as the noise would be additive. You could try swapping tubes to make sure it's not a tube. Remember when you use the phono stage you are also using the high level stage as well.

If the level of "hiss" does not change with the gain controls and the same noise is present on all inputs, it is virtually certain the problem is not tubes. Both channels/equal noise also means the difficulty is very likely in the power supply, which feeds each channel equally.
Also be aware that some (however little) noise will be audible normally at zero gain unmuted.
Be sure you are hearing a "hiss" and not a "buzz" (which could indicate a ground problem). Tracking down ground and ground loop problems can be a royal pain in the low end.
If nothing helps, don't hesitate to get it checked out by a reliable technician. You can get reliable recommendations for one of these invaluable persons by asking right here on audiogum.
Good luck. It's a fine preamp.

It seems like a no brainer. Your tubes are used up and/or have always been noisy. This unit requires low noise AND rugged tubes. It has a reputation for that which precedes your problem.

FWIW, in light of other advise to the contrary, the line stage follows the gain controls. The volume controls only attenuates the volume from the sources before it gets to the line stage. Any noise from tubes or anything residing in the sources, including phono, would be changed by rotating the volume controls, but noisy tubes in the pre-amps line stage would always be constant. The extent that noise from the line stage would be heard would depend on the efficiency of your speakers, input sensitivity of the amp, and of course the quality of the tubes.

Most owners of this unit end up using something like NOS 6h23EH which are both rugged and (usually) very quiet. FWIW I have identical issues with my ARC SP10 and ended up using 6h23eh in the line stage tubes. Unless you get the right tubes in the first place you can easily burn them up in 100 (or less!) hours, and you can easily burn up a lot of dough trying find tubes that will perform right in this pre-amp.

This is any old and often discussed issue with the AI. Do some research in the forums and you'll find them.

Hope that helps a bit.