JAN Philips/Sylvania 7581A vs. Golden Dragon Retro

I purchased an integrated amplifier equipped with Golden Dragon Retro KT-66 output tubes. Nice amp, but even after a substantial burn-in period, it has a somewhat "cardboard" and unclear midrange which I attribute to the output tubes.

Are JAN Philips/Sylvania 7581A direct substitutes for KT-66 tubes? What is the tonal characteristic of JAN Philips/Sylvania 7581A tubes?

Any opinions and recommendations will be appreciated.