Hello,

The EV of standing with BJ against 6 is +1.50 and the EV of double down is +0.54 for a full deck and typical online rules.

If you remove all fives your EV is +0.77
If you remove all aces your EV is +0.80

But if you remove all aces, fives, fours, sixes and threys then your EV will be +1.75 and you should double down! And in that case your EV for playing is almost +5%, which is both very good and unlikely...

/ Johnny