I have a cherry and a black one (yes, I do not collect for fins, I know, but the flesh is sometimes weak). The black one drove me nuts with its lack of sub bass, so I had eventually a mudbucker in neck pos stuck in, you can now play dub with it alright!
Cruel to be kind:
The cherry one - for whatever reason - delivers over an Ampeg SVT rig with fridge and an extra 18" cab, in neck pup mode only (loudest setting of the mini toggle) and bass boosted on the SVT via the boost button
just enough bass to get away with it (the black one never did prior to the implant). Still considerably less than a TBird, P Bass, Jazz Bass or Ric would, none of which is a sub bass monster. An no comparison to an SG RI or Standard either (but then those have close-to-the-neck pups)
I guess it's a couple of things: there is little wood on the bass (thin neck and body), here a comparison of SG-Z body and neck with my one off SG long scale (pre-owned by Herr Carlston, apologies for the awfully glary pics, I did this in a rush, the SG-Z is the one in the lighter cherry, an unusual tone I haven't seen on any other Gibson),
the pup positioning is extremely towards the bridge, farther back than on either a TBird or an LP Bass, again a glary shot to compare with the one-off SG:
the pups are built for clarity (with a nice high middish peak) rather than ooomph and the toggle switch passive electronics seemingly deball the sound even more.
Other than the lack of deep bass, the basses sound fine and those fidgety toggle switches can give you lots of nice mid range nuances, I'm sure they would have made great fretless basses where the lack of deep bass is not so detrimental to general usability.