I believe your grounding scheme is not efficient enough. It's kind of a common problem here, as it's usually a first project for most of us. You have to think of the ground as a return path for different signals, and not as a "neutral" point where everything gets nullified. Ground is nothing more than the opposite of the power rail, after all. A perfectly silent tube amplifier uses a balanced wiring for V+ and Gnd throughout the amplifier, with proper decoupling at each stage.
The voltage you get at any single point is in reference to the ground at the other side of this point. If that ground point is noisy, the end result is the same as if your V+ was noisy.
Power supply grounds are noisy. Signal grounds are sensitive to noise. Therefore, make sure the power return paths are not going across your signal return paths.
This is why the first recommendation most people have is to tell you to use a Star Ground. Connect all your return paths to a single point. Connect this point to the power input jack using a single wire. Connect the star ground to the chassis at a single point, making sure no current flows trough the enclosure. This is not a car, it's an audio device. The enclosure should be used as nothing more than a shield.
I had to rebuild my whole amplifier many times to learn all that. Now it's dead silent.
