It has been my experience the new base gaskets are easy to install slightly off kilter and leak. When I replace cartridges now I reuse the old base gasket and save the new just in case.

A little residue from the old gasket or hard water deposit is frequently the culprit. Taking extra care to ensure the mating surfaces are as clean as possible before installing the new gasket is well worth a couple of minutes effort.

Thanks for the suggestions. I took the handles off again and tightened down the cartridges a bit more, which seems to have helped. Is it possible to overtighten them? It didn't take much to loosen the old ones, so I initially didn't do much more than a half-turn or so past finger-tight. I was able to do at least that when I went back again. The leak seems to have stopped, so I'm thinking I was probably just a ninny. Thanks again!