i have both. i bought the jazz specifically to have that "jazz with flats" sound. i'm not a fan of zingy top end.
the t-40 is obviously going to have a helluva lot more tonal options than the jazz, and in my opinion the jazzenbacker thing the t-40 does in SC mode is a lot more enjoyable to *my* ears than the jazz.
both are almost annoyingly versatile. however, the t-40 essentially slaps the jazz around in "all around versatility."
jazz basses have always [because of their pencil necks] gone out of their way to warp for me; no matter how much i tell them i don't appreciate it.
where, with the t-40, i've contemplated punt kicking it down the my apartment buildings staircase; but won't, because i would feel bad for the stairs.
there will be snooty people that think that the t-40 isn't as good/versatile as the jazz. you know those people... "i have a $2,900 boutique jazz copy that i waited 19 months for and when i got it home i slapped a brand new j-retro and a couple darkstars in it. i just
set all the controls flat, and its the most versatile thing evarrrrr!"
really? that one setting is somehow magically sonically more versatile than a passive system with sc/hb mode, series/parallel switching, and seperate volume and tone controls so you can you the three way switch to change between two totally independent sounds on the fly and combine the two signals? really?
i used to be a "P into Ampeg" kind of player. however, reading some of the reviews on the t-40 had me pull the trigger on one and honestly, despite a bit of a learning curve i'm now really happy with my t-40, jazz, and PJs. once i learned how to set the levels on things i realized how much i was missing out on. the t-40 will never be a jazz but it has a slammin' character all of it's own that i personally wouldn't trade for a multi-thousand dollar bass of any other kind.