What tubes to try on a Grant Fidelity tube buffer?

Hello, I recently purchased a Grant Fidelity tube buffer to try, model B-283, it uses 6J1 tubes. I'm new to tubes and curious about what other tubes to try. Nothing wrong with current tubes, just don't have anything to compare them to.

It's installed in between a Denon 3802 used as a preamp with NuForce 9.02 monoblock amps on ProAc Studio 110 monitors. Would like the end result to be detailed yet musical, and just a tad warmer. Thanks for the info.