Toilet flapper still leaking after 2 replacements?

I've got a toilet flapper valve that's still leaking after replacing twice, with two different brands of flapper. It's a slow leak - like "drip-drip" every 10-15 seconds (I guess I could be more scientific about it and turn off the supply to see how long it takes to drain the tank . After the first replacement continued to leak, I scrubbed the valve seat with a scotch-brite pad to make sure there wasn't any mineral build-up, etc, but that didn't seem to have any effect

It just occured to me that before replacing the valve the first time, I had the tank off to paint the wall (the original valve had been leaking before that). But I assume that if that was the cause of the leak, I'd be seeing water on the floor, not leaking into the bowl?

What's my next step - replace the valve seat assembly?

Turn off the water to the tank, as see how low the water goes.
If it stops at the flapper, that it
If it keeps dropping below that, it could be the seal between the tank and flush valve.

Or if you have the fill tube from the fill valve too far down the overflow tube, it will siphon water from the tank.
Fluidmaster and Korky have little metal parts that hold the fill tube above the overflow tube to prevent siphoning.

I had a leaking toilet flapper and the one I used to replace it came with a glue on seat that the flapper sits on. I emptied the tank, dried the seat area up , cleaned the area where the seal goes and glued it on with the supplied silicone. Let it dry and it has not leaked since. If the flapper is leaking you can often here the water trickling past it. Very annoying if your bedroom is near the bathroom like mine is and your a light sleeper.....