I think doubles are a questionable way to evaluate really any player: As pointed out here, the rate at which a player hits doubles varies widely from year to year, such that the number of doubles the player hits in any one year isn't much of an indication of his overall talent level. If Votto's (or any player's) doubles are down this year (or any other year), I'd suspect it's due more to year-to-year variation than a legitimate decline in power.