I loved 6V on a 901, but they leak a lot, (A LOT) so I gave them up for 808 cartos. Regular ones, not HV or LV.
They're working well, last forever, and I've had no burnt issues.
I add 8-10 drops probably every hour and a half. I take off the cap and drip in. The "condom method" can get liquid in the center and makes for a very difficult draw and liquid in your mouth.
Keep them very moist.
Maybe it's your liquid. Is it good tasting at 6V with another atty?
I'm a Dekang fan, but lately have been experimenting with some flavors from Vapor renu and Tasty Vapor. Both are excellent @ 6V.