In the past (before the wood body Grados appeared) we ran the loading on the Grados at anywhere from 7K to 10K with excellent results. I can't speak for the current lineup, but on the lower priced models I would not be surprised in the least to find that 10K is still a good value.
At the very least I would set the Yamaha to 30K. A little should be better than nothing, although our experience was that you really didn't start hearing anything until you got below 20K or so.
At the very least I would set the Yamaha to 30K. A little should be better than nothing, although our experience was that you really didn't start hearing anything until you got below 20K or so.