Both the set of 300B-98B and the 274B rectifier arrived today. I installed the 300B meshplate tubes in the Pacific DAC, and let them settle in for a few hours. Compared to the KR-PX4, the 300B is cleaner on the top and has more "punch" to dynamics across the frequency band. Importantly, the microphonics of the PX4 are gone. Bass especially is richer and more dynamic. I compared the 300B mesh plate with 300B solid plate tubes. The mesh plate are a bit cleaner on top, removing a touch of glare versus the solid plate variant.
The above observations were made while still using the KR-5U4G rectifier. Late in the afternoon, the Psvane Acme 274B rectifier arrived. Replacing the 5U4G with the 274B made a huge difference, more than installing the 300B meshplates.
The whole frequency range is more controlled and with more exciting dynamics and illumination of timbre (more complex and easily identifiable signatures to pianos, horns, strings, etc.). Definitely a step up the realism ladder. I wasn't expecting the level of improvement from the 274 rectifier. Just shows how important is the power supply.