I wanted to pass along my experience with the TDSS upgrade to the IDA-16.
First, I want to reiterate two commonly experienced points about the IDA-1. It is dead quiet, and it is neutral. When a strikingly low noise floor is added to tonal neutrality, the result is that you simply get whatever is there in the recording. If the recording engineer decided to add "air" with the digital equivalent of a "hot" miking, well, that's what you get with the IDA-16. Back in the day when I wrote reviews for Positive Feedback, I used Rosanne Cash's 10 Song Demo for the simple clarity of its nuanced presentation of her voice, but then also to see how a new piece of gear dealt with the hot miking of its opening track. The IDA-16 on its own just passed it along through my Selah Audio Extremas, the question then became what would the TDSS upgrade do to the inevitable glare built into that recording.
Well, at first, indeed for more than 200 hours of listening, the TDSS upgrade didn't change the presentation of that glare one way or the other. Then after those 200 hours, I began to notice two things: the noise floor seemed even lower, which I honestly didn't think would be possible, and then about an hour into each listening session, the whole gestalt of Cash's voice became more present as, well, just plain *her* voice. ---And, yes, that is exactly what you want from an upgrade in the high end, you want the music you love to become noticeably more itself in all its nuanced details.
Up until those roughly 200 hours of listening, I had begun to wonder if the TDSS upgrade was really worth the money (which is honestly not that much given high-end prices nowadays). But as I come into 300 hours of listening, yes it clearly is. You just have to be patient to hear the virtues of the mods that Bob Smith has worked out for the already wonderful IDA-16.