I agree that the K500/K501 sound better than those but they are kinda rare now. I always see a listing pop up on here and when I jump on it it tends to be already sold. I would really love to own a K50*
Also you are right about the K501/K601 sounding different. They sound nothing alike. All the old vintage AKG's are voiced far differently than they are now and to be honest I prefer how the older models sound. Now though I still love all the current models and they are still great.
tdockweiler, You should never go the the FRG. Those a lot of the time are not even accurate at all.