To the OP, "looking for better sound stage & imaging , dynamic with reasonable accuracy", and following your other notes, I can vouch for the J2 giving fully what you seek, in my case, using transparent 104dB single driver horns. The J2 gave slightly more separation, layering and great organisation of music than my treasured AD1 4w + 4w SET, just without that certain valve quality that in the end I preferred. (Given an opportunity I would try an FW SIT3 or F8)
The minimalist design principles of First Watt passes on more musical information by design, and so, I wish you luck finding the loudspeaker system that synchronises with this approach to your satisfaction.
