Is it necessary to ground a turntable? With or without it grounded I get a hum at mid to moderate levels. What do I give up by not grounding it? Thanks.
Something is near a power line. Try moving your gear around.
I, too, have found that grounding turntables to preamps (line or phonostage) doesn't always ameliorate hum problems. (Sometimes it can even exacerbate the problem.) I would certainly try what Sunday has suggested and move gear to different locations to keep the low-level-signal-bearing interconnects that run from your 'table to your phonostage as far from power transformers as possible. You might also want to try "braiding" (intertwining) these interconnects that go from your turntable to your phonostage. That can sometimes knock down hum, although this doesn't always work and rarely completely eliminates hum.