The + upgrade to the Ultra preamp involves replacing the op-amp chips with newer ones in existing sockets, and can be done by yourself if you're careful. It is low cost, and results in noticeable improvements to your preamp's performance. There are a number of threads discussing the results, and also comments in the stickied announcement of the upgrade.
The + upgrade to the Ultra 550 amp replaces components on the circuit board, readjusts the circuits, and is done by AVA. The cost is $200 plus shipping, and is a refinement in the highs and lows. In an A/B comparison between the Insight and Insight+ amps, the results we heard were even smoother highs and tighter lows. i'm certain the Ultra+ exhibits the same improvements over the Ultra.
Has anyone had a chance to compare to the two? Preamps or amps? I'm wondering what differences there would be if I upgraded my Ultra SL preamp and/or Ultra 550 amp.
Edit: See Lefty's reply #3, my comment didn't include the latest upgrade developed.