The RT 60's spectrum is more important than its total value. The best-sounding rooms have an even distribution of spectral energy that slopes down as the frequency increases and a very even response on the low end.
The current SOTA mastering room design is to highly control the energy to be very even with a short decay. It is an almost anechoic room where skyline diffusers have been strategically placed above the listening position and side walls. This generates a bit of even and diffuse liveness in the area of the listening spot but leaves the rest of the room echo-less. Low-end control is usually incorporated into corners, the ceiling, side walls, and the back wall, with sometimes complex construction techniques to even out the low-end energy.
I've worked in several of these types of rooms, and once you get used to them, the amount of information, especially imaging properties, is jaw-dropping. The super low level of ambient noise is somewhat disconcerting as well (when music is not playing), but that just increases the perception of detail.