Sean- Since phase shift (time delay) plays havoc with the harmonics of a fundamental note, a better question might be, what does NOT cause shift. Answer- Speakers with first order crossovers (e.g., Vandersteen, Thiel, Meadowlark...) and DSD (SACD) coding / encoding. That's all you need to know IMHO.
What causes phase shifts...Sean?
Also, can anyone recommend a good resource on the subject?