The grid stopper should generally be anywhere between 50 ohms and 10K ohms. Sometimes different values appear to give different results, but the available reports are anecdotal and subjective, so it's hard to draw conclusions. Fortunately, it's easy to experiment using your own ears, which is after all the only measurement you can really trust!
As a scientist I am deeply skeptical about what your ears tell you. As a music lover however, I say if it sounds good it IS good. When in doubt I have to go with the latter. :^)
