[brucek]

All that NP walls do is make identifying and taming room modes more difficult

That's the first time I've heard this. It's not that I doubt it, but my understanding is that 4 non-parallel walls don't allow any standing waves a chance to build.

Here's a small article that fits nicely into my novice knowledge of the subject.

Non-parallel walls

RR Audio designs enclosures based upon the fact that parallel walls cause standing waves inside the enclosure which cause certain frequencies to predominate at wavelengths based upon the distance between the walls. These frequencies and their harmonics will cause reinforcement (or cancellation) which causes peaks (or dips) in the frequency response. Depending upon the bandwidth of these resonances, there will be energy storage and ringing at these frequencies to some extent (a great deal if the resonances are high-Q). By designing the enclosures with non-parallel walls, the wavelengths between opposite walls are different at any two adjacent points, which spreads the frequencies associated with these wavelengths and eliminates standing wave modes. The reduction in midrange distortion and frequency response anomalies is significant. In addition, the rear-wave energy from the LF driver is bounced on paths which do not immediately re-contact the diaphragm. The energy is bounced all around the interior of the enclosure, encountering damping material at each wall which absorbs much of the energy, so when it finally recontacts the diaphragm, the amount of energy is greatly reduced from the original level. This reduces the phase distortion caused by rear-wave energy creating vibrations in the diaphragm which have no relation to the current audio signal, and substantially improves transient detail performance, low-level detail retrieval, and clarity.

brucek