Grounding adviseable on tube pre-amp powered by a 9VAC power brick?
The PSU does not need to be grounded, it is intended to be AC to avoid ground loops (if it is grounded then that is an opportunity for conducted noise along the ground to interfere with the audio setup.
The amp should have filtering to reject noise on it's power input, if it doesn't then there are probably better options than getting a new PSU.
One of these things would be to use a line filter to filter out anything but the fundamental frequency.
Another thing would be to make sure the noise isn't coming from other sources in your audio system, and make sure it is the power source. Ferrites on cables could cut back on high frequency noise if the source is something like cellphone radiation.