Neck pup only = you can approximate a TBird sound
Bridge pup only = raunchy mids and little else
Both together = things get a bit P-bassish minus the clarity
In fact, the sounds are pretty variable, but regarding treble and especially presence they are all a little muffled. I've tried to combat that with especially bright sounding strings such as DiMarzio Pro-Steels, but to no avail, the pups just can't transmit any type of presence, though the Pro-Steels helped with the treble on my Nikk Sixx who has pehaps a bit more high mids than a regular Epi Bird would have. No matter what you do, you won't be Geddy Lee or Chris Squire though. But it's not a structural issue, my silverburst Epi TB with Gibson chrome TB Plus pups has the same amount of "shiny, but never glaring" presence a real Bird would have, it just doesn't have the neck thru sustain (nor could it, being bolt-on).
But I remember a test in a German muso mag where they compared Epi Explorer to Epi TB side by side and they in principle surmised: "For a dark vintage Gibson sound, the Explorer is your bass, all others who want to be surprised how versatile a bass with a TBird look can sound are better served with the Epi Thunderbird."
Versatile without presence that is!