We have a Phoenix brand tub faucet, 8", replaced seats and stems, still dripping out of faucet. What can be done to fix this?

They only thing left here would be the valve seat itself the washer sits on to seal.

HowStuffWorks "How To Repair a Faucet Valve Seat"

Top 10 Plumbing Tools: Faucet Valve Reseating Tool

Pull stem and look at the seat it sets on to seal and see if that looks rough and needs grind-ed.

21boat nailed down, Before you reseat the seats I would like to add my 2 cents, shut off both cold and hot angle valves under the sink, remove both stems open the angle valves for a second to flush out any debris out of the faucet have a towel ready to catch the water, install the stem and check if the dripping is stopped, good luck.

John