Showers also sometimes relieve my tinnitus to some extent as well.I've also noticed that the only thing that I can definitely say that has an effect on my tinnitus is having a shower (I have mild intermittent tinnitus with mild hyperacusis). I can wake in the morning with tinnitus, have a shower, and (on some occasions) the tinnitus disappears for the day. If I don't have a shower, this never happens. It's the only thing that has an effect. This has also been observed by others (e.g. @Golly ). Playing shower noises has no effect.
Now the shower provides both auditory and electrical stimulus (in the form of the sensation of water drops on the face and neck). I think this is a crude form of bimodal stimulation and that's why it works. Which I guess is an encouraging as far as bimodal stimulation goes. Maybe if you had 2 x 30 minute showers everyday this might helpful!
The way you expressed it, as bimodal stimulation, actually seems pretty insightful. For all we know the electrical stimulation from experiencing warm water droplets all over your head, face, and neck is not crude at all, but effective exactly because it is so saturating.