The findings from the rehearsal with the Ultra Pickup are surprising and it shows once again that it is difficult to reliably evaluate such a change at home.
In my constellation (band, musical material, my equipment) the following emerged:
The Ultra pickup doesn't really harmonize with the MM pickup. The two sound very different. The Ultra is something like a vintage pickup with an extended frequency spectrum and more volume. While playing I switched back and forth between the Ultra and the MM (front coil, parallel, rear coil) and with the Ultra the A and D strings disappeared in the mix. You would have to play the Ultra with different amplifier settings, which would then not be suitable for the MM pickup.
I'll put the Model J back in and it'll stay that way.
The DM Ultra is undoubtedly a very good pickup, but not in this / my configuration.