Audible Illusions hiss

Since you're hearing the same noise level(I assume) in both channels, it might be the time to replace DC caps. How old is the unit?

Sounds like your line level tubes are going bad. My 2D is pretty quiet. Roger at Music Reference has some robust and quiet tubes for a reasonable cost.

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.