I already wrote it in the ZA3 comments. The opamp development brought excellent results. The stage image became much more transparent and wider, and the dynamics also increased. The Sparkos Labs SS3602 is great, it has greatly improved the category of the ZA3!
SparkoS Labs SS3602, MUSES02, OPA2134PA, High Quality Operational Amplifier Chip for BT20A PRO and V3 (1 Pair 2PCS)
SparkoS Labs SS3602 was a worthwhile addition for my sound level
While the opAmp upgrade cost more than the BT20A Pro, the sound quality improvement is worth it. Would have been great if shipped with instructions, and if it arrived a bit sooner.
Ordering and delivery was a pleasure experience. It comes as 2 per order.
With these opamps installed on my R and L channel, the overall sound profile became warmer. The highs got smoother. The listening experience became more enjoyable for me because it tookout the listening fatigue I was getting with the stock ZA3+Airmotiv B1+ setup.