I have a few thoughts about this competition.
1. It's a great idea but I wonder if the prize is too exclusive for people to want? If I won a prize of that value what would I do with it - I enjoy drinking whisky but at that value, even though it cost me nothing, I would be disinclined to do that, so that just leaves keeping it as an investment or selling it on, neither of which are my particular interest.
2. I can see that the premium competition has to provide a better possible prize than the normal monthly one, but obviously more entries are going to be achieved on the free to enter than pay to enter. Perhaps the premium competition has to be a lower value whilst exceeding the free competition or instead of one prize that may be unattractive to some, like me, you make up the high value with a bundle of prizes, e.g. two $500 bottles or three $300 bottles or a prize valued at a lower price and build as the popularity increases?
3 Perhaps a "whisky voucher", not sure if there is such a thing, that the winner can use, the value of which is established by the amount of subscriptions made during the specified time period. That way those who do enter know exactly when they will see a result.