But I do believe strongly that the pop comes from your external DAC
I've now had a chance to run tests not feeding A16 optical output to my external DAC (with its own amp volume set at 46 out of 47) and XLR to my Stax amp (set at volume 2 o'clock), but rather using the A16's built-in DAC/amp and RCA headphone output (thus acting as a "preamp") to feed RCA input to my Stax amp.
And I can report VERY DIFFERENT RESULTS as contrasted with the previous list based on optical output and my external DAC.
Turns out NONE of the previoulsy reported noises (i.e. Crack1 - Crack6) occurs when using internal DAC/amp and RCA headphone output. Most importantly, there is NO Crack5/Crack6 when either directly selecting a preset via 1-9 or sequentially selecting a preset via +/- ADJ. And there is no Crack4 at power-off. And there is no Crack2/Crack3 when "active" displays on the Home Menu screen for UserA and UserB presets. And there is no Crack1 when "wait" appears on the Home Menu screen for UserA preset.
But... regarding using internal DAC/amp and RCA headphone outputs... there are instead three new minor "pop" sounds which occur during power-on:
(a) Pop1 occurs IMMEDIATELY and INSTANTLY on power-on when the Smyth logo is presented brightly. Probably from powering up the headphone amp.
(b) about 15-seconds later, right when the Smyth logo goes dimmer, two short minor pops Pop2 and Pop3 occur, one right after the other
(c) no more pops or cracks or sounds of any kind, including when "active" appears on UserA and UserB presets of Home Menu screen, including at power-off, and including when selecting a preset either directly or sequentially. So the rest of normal full operation has NO ANOMALOUS SOUNDS WHEN USING RCA HEADPHONE OUTPUT. Totally "silent" operation right through power-off.
It seems clear that the previously reported anomalous crack sounds are directly tied to whatever the A16 is doing driving its optical output to feed an external DAC. I have reported this second set of test results to James, so that they can resolve the overall issue completely.
From James: "Good news, Stephen has come across the same problem and says it will be fixed in time for the next update which is due next week."