A few weeks ago I ordered Mach IIs for my Super J and my 5-string ABII. Two days later they were at my door and I installed them right away.
Sheldon's description of the sound is accurate: "It has a hotter output with punchier, more present lower-mids."
At first I was wondering if I had made a mistake because I was satisfied with the sound I had before. The "more present lower-mids" sounded like boxy upper-mids on my cheap little 50-watt practice amp with a single 10-inch speaker. I was able to reduce the boxiness (I call it the cardboard sound) with EQ and get a really good barking sound out of both basses.
The other day I took the basses down to a music store and plugged them into a Mark Bass 450 with a 4X10 cabinet. I know that amp can make anything sound good, so I did A/B comparisons with another ABII with all the EQs set to flat.
This was where Sheldon's description really rang true. What was boxiness on the cheap amp was a really solid, punchy sound with lots of bark on the serious amp. I'd love to hear this pickup on a fretless. The original pickup still sounds good - just mellower in comparison.
The higher output of the Mach IIs make them a little more susceptible to RF noise, which was much more noticeable than I remember before. That's at home, though, in a room with a computer, TV and compact fluorescent lighting. In the store, which was pretty quiet a the time, I didn't hear any RF noise at all. Sheldon said he might add a some kind of simple mod to the circuit to help with the noise.
While I like both the new and the old bridge pickups on the Afterburner (I think I'll leave the original in my 6-string AB1), the Super J version is a must-have for me. It is a much better match for the P-style pickup than the previous version. I never like to combine P and J pickups on any kind of bass - I like both styles individually. The Mach II J blends with the P better than I have heard anywhere else before. I'd like to hear comments from somebody who has a Super JJ about the those two pickups combined.
On a side note about the debate between JJ and PJ basses: PJs give a variety of sounds from two classic basses but what they can't give you is the neck sound of a J (not too bad) but more important, they can't give you the Marcus Miller JJ in parallel sound, which is my favourite slap sound - even though it doesn't cut through as well as others.
Also a little off-topic. The store didn't have another Super J to compare with but they did have a Korean-made Lakland P-Bass (a Glaub, I think) with a Fralin pickup. This was the first Lakland and Fralin I have ever played and I really liked its sound. A good solid classic P-bass sound that modern-style Fenders just don't have. Well, as good as that big bass sounded, my little Super J with the Dingwall P blew it away. It has all the tone of the Lakland/Fralin but with better clarity and definition - and it still sounds "vintage" when you want it to.