Well that's pretty significant. I was originally suspicious of the pressure being responsible for soft bass and discussed it with Bruce @ ET. He said no it would not cause that unless pressure was very low, and that the *apparent* reduction of bass was actually more accurate to the source compared to a pivoted arm, which tends to build up bass due to the summation of vertical and lateral arm resonances. The ET arms don't sum those resonance points and therefore paint a much cleaner clearer representation of the original.The higher pressure gives a stiffer air bearing. Sonically, the entire presentation solidifies. This is especially true of the image. Bass, thought to be a weak point of the arm, becomes a strong point. The sound stage gains more specific qualities where each instrument can be placed with certainty and the walls and dimensions of the venue become more than abstractions. And the primary image becomes holographic. Other than that, not much.
Still I have reservations about that taken entirely at face value. Your description is exactly what I would expect from a stiffer pressure. The question is, though, at what point (psi) does it become significant, and above what point (on each manifold type) is further increase irrelevant.
Separately, I found that the biggest factor controlling apparent bass on my arm was the correct matching of mass loading between the arm and cartridge. The manual is not at all clear in that regard, and seems to gloss over the significance of matching the two.