Wow really strange spot but I would call without even tanking because reg speak before fish and I feel it's really unlikely he check turn with 33 or KK. Plus your line can rep a nuts flushdraw that goes for a c/r for value river (since nobody really look like to have full/quads here and villain think he can bluff you this deep because the board is paired. 

So I really wouldn't fold, it's the best combo you can have here and I don't want to fold 100% vs a range that can turn some hands into a bluff. His value range that beat you is so small that we really don't need him to bluff a lot of combos to make it a call.

Here's my take: 

While Villain probably SHOULD be flatting KK facing an UTG open, he likely 3bets more often than not. Let's be generous and say he flats 50% of the time. So right off the bat we can discount 3 combos of KK. 

He probably cold calls 33 every time (even though I suspect it's -EV), so nothing to discount there. 

I think we can discount some additional combos of KK/33 when Villain checks turn. With the fishy player behind him and you checking, he should bet turn some portion of the time, esp when effective stacks are deep. Maybe we assume he bets here 25% of the time when he holds KK or 33. 

After all the discounting, we're left with only 1.5 combos of value hands. If Villain is ever spazzing out with 66, or bluffing with a bare Ad or Kx thinking it blocks some nut combos, you have an easy call. 

I completely agree with what both of you guys have said, essentially because his value range is so minute we have an easy call because he doesn't have to be bluffing here very often in order to make this call profitable. My only problem is I really don't see a reg at this level bluffing in this spot at all, they generally never take a high variance play and given that I think he is never bluffing, which is a big assertion even though his value range is minute that's all he is ever gonna show up with so I think it is a tough fold.

Zero% chance he`s bluffing here.It`s 33, KK exclusively imo.

Checking turn is a better play for villain then betting.Betting would fold out all of fish`s middle pairs, maybe oe/gs`s as well, so he prolly only gets value from flush draws.

He also folds out all of our our air type hands that cbet paired board + 77-QQ a good portion of the time.They have 12 outs combined that he gets value from (stacks them) if they hit otr, and might still call a bet unimproved on some bricks.

I agree with what has been said thus far it seems very unlikely he is bluffing and any half decent reg does not shove 66 here. Curious about your plan on the turn though as it seems unlikely reg ever has Kx because he would have to float flop pretty wide with fish still behind so he can only really have Kxdd of which there are not many that flat UTG+1.

 Both reg and fish are more likely to have pairs or flush draws and by betting we can get at least one more street from fish or do people think checking turn and then making a thin vbet on blank rivers is better?