Mark,
This is an interesting, if esoteric point. Smaart's delay finder cues in on the broadest early area of consistent group delay arrival. It is not measuring the true T0. The way you would measure "true" T0 is to look at the phase behavior in the stopband of the loudspeakers HF rolloff, where the group delay will be essentially constant. This is not particularly easy to do. There are other ways to do this, but they are not implemented in SMAART.
The apparent phase behavior near Nyquist is, of course, shaped by the granularity of the reference delay time. Those skilled in the art don't expect the phase response at the Nyquist corner to be be flat, but rather flip-flop between lagging and leading based on a change of one unit in the reference delay granularity of the DUT.
All discrete time measurement systems, be it with "stochastic" or "deterministic" sources have the limitations of the phase resolution set by their sampling rate. If the DUT has a lower sampling rate than the measurement system, the phase can be determined more accurately at for the DUT.
