Doing a poll is good to see what Player want, but that is not the most efficient way to choose a HA.
That is a very high and strict position which not everyone can handle, let's say people vote for a player who is temporarily the most active but his experience with our server is too low. Poll may ruin the server's administration, however that doesn't mean that it won't be a success 2.
I'd say Do a poll, just to analyze what players think, and afterwards High staff should analyze all members included in the poll and choose from them. This doesn't mean that the player who has most votes will be HA nor the one who has less votes will fall out in first time.

Hope you understood what I meant,
Who ever you choose, just be sure that he won't leave and that he will improve the server.
Cheers,
Popay