In the last two months, my 2000 Thumb NT4 (fretted) has been improved from its factory-stock state.
One of my bands rehearses at a studio a few blocks away from NYC's Empire State Building's radio transmitters. The Thumb was picking up lots of radio interference. That never, ever happened before, but in this studio it was unplayable. None of my other basses had the problem there.
Turned out that the paint in the control cavity wasn't conductive and the back cover was unconductive plastic! So my tech pulled out all the electronics and lined the control cavity and cover with copper tape. Problem gone!
And then the MEC 18V 3-band EQ pre-amp was as noisy as could be, hissing ridiculously. I replaced the two op-amps with lower noise ones. It's much, much quieter now, at the expense of reduced battery life. That's a trade I'm happy to make.
And the Thumb has had some other maintenance too. A few months ago I sent the bass down to Warwick in Nashville TN for some work: the neck had a bow and a slight twist that the truss rod couldn't remove. They used a heater to re-curve the neck and now it's playable again.
Love my Thumb. It's my #1 bass.