Hi Spocker, don't have a IB6 Bass but reckon they are the same units as used in IB5 and EX5 where the cavity is 105 mm x 40 mm.

The stock units don't sound too bad and well worth trying them out before shelling out $200 to $300 on upgrades where the choices are mainly Bartolini, EMG or the Custom Bensons that I installed in my EX5. There are sound demos in my build thread, just click on the link to Build 1 in my signature.

Cheers, Waz