After a few minutes of noodling around, some observations:
I dislike new rounds. I'm so used to flats that my finger technique creates a LOT of string noise on new rounds. I may have a set of Labella flats around, and if so they'll probably go on the Thunderjet.
The action is REALLY low and nice on this bass. Minimal effort is needed to play it.
BIG sound. The room was thundering when I tried some Allman Brothers (Statesboro Blues) and ZZ Top (LaGrange) tunes.
The electronics on this bass don't stress treble. I can get a reasonably bright sound, but clearly the Gretsch ThunderJet is made more for the mids to the low end frequencies. Even at full treble setting, it's noticeably less bright than some of my other basses. IMO this is A Good Thing.
Nice balance. I put my best sheepskin-padded strap on it (visible in last photo above), and it's a nice package.
The ThunderTron pickups on this bass are hotter than either of the other two. That's interesting because my 5123 also has ThunderTron pickups, but it has the lowest output of the three.
Ranking the highest to lowest output from the Gretsch Brothers, it goes like this:
Hottest: Thunderjet 6128 (Scale length 30")
Middle: Gretsch 1222 (Scale length 30")
Low: Gretsch 5123 hollowbody (Scale length 32")
I have more learning to do with this bass, but when I spend $1000 for an instrument, I intend for it to be a lifetime companion. I will play it and report more later...probably this weekend.