I have spoken with Dave several times and made a few autoformer preamps for friends, using his and Pieter's autoformers.
Transformer passives have more bass distortion as compared to autoformers. This is easy to confirm: look up the low frequency response graphs at S&B and Sowther sites, also, Jensen line input transformer data sheets, including % of harmonic distortion at low frequencies. A magnetic core will saturate and distort with a combination of very low frequencies AND high input signal levels.
OTOH, a transformer's galvanic isolation is great to get rid of ground hum issues, it also attenuates very high-frequency digital noise such as the normally unfiltered SACD outputs or out-of-band DAC artifacts.
IME, autoformers have great bass, outstanding headroom and can be made to roll-off smoothly at ultrasonic frequencies.
In summary, IMO, if your system has ground-induced hum, or an SACD player or your CD player outputs digital HF nasties, use a transformer volume control.
IMHO, if your sources are clean of HF noise and there is no ground-induced hum, an autoformer is a superior solution across the board: bass, dynamics, low level detail, frequency response, distortion AND musicality.
My 1c worth (adjusted for recession)