For what it's worth I'm going to be building Tom's Modulus-86 next.
How did you level match for your listening tests? Did you match within 0.1dB? Were the tests at least semi-blind? How did you manage the psychological influences? You apparently expected the Pass ACA to be "distorted" as in "harmonically rich" and it was.
Your comments reminded me of my reactions to retail salon high-end system auditions in the 1980s. I found many demos in that era to sound dry and analytical as audiophiles and hi-end audio salesman thought they were emulating "live" vocal and small ensembles. In reality, most of those recordings were made in dead silent studios with completely different acoustics than the venues where I actually experienced such music. To this day, I take issue with audiophiles claiming that such recordings can be made to sound "just like live" in the sense of a real world live performance with an audience.
Many such recordings can create an illusion of the performers being right there in your listening room, but that illusion is created by talented and skilled recording and mastering engineers who can be "artists" as much as the musicians themselves, because they are the ones who actually "create" the recordings. I have experienced many listening sessions - at my home, in stores, and at the homes of friends - that create an incredible feeling of "being there." But it was never the same as going to a small venue and listening to live performances reinforced with Electrovoice, Peavey or similar monitors.
The closest I have ever come to live and recorded sounding similar is with live events in small venues with acoustic instruments and no electronic amplification. And no audio system has ever reproduced perfectly the concerts I have experienced over the years by attending the Chicago Symphony Orchestra, San Francisco Symphony Orchestra or S.F. Philharmonia Baroque. Or the acapella men's group, Chanticleer, singing in a small historic church, a California Mission cuurch, or a cathedral (where I sat next to the group's recording person, who was in the aisle seat about 10 rows from the front, using a digital tape recorder and a pair of microphones mounted on a skinny pole about 8 feet above our heads).
I
walk the line between objective and subjective by accepting the objective as "what is physically there with respect to an audio signal or sound waves," and the subjective, which is my personal experience and/or "perception." However, I
draw the line at claiming that differences in my "perceptions" are representative of the character of the actual sound waves that vibrate my tympanic membrane, or claiming that I can hear physically non-existent "differences" in sighted listening comparisons, differences that scientific research has demonstrated cannot be detected.
It will be interesting to hear about your experience with the Modulus amp, both about the assembly process and your subjective comparison of the sound to that of the Pass ACA. Unlike DAC's however, which shouldn't produce audible differences if properly designed and manufactured, the "sound" of those two very different amplifiers will very likely be influenced by their synergy with your speakers and your room.