I believe Dan is correct regarding magnet strength. I think the IIIs have less strength and slightly warmer/bassier than the brighter IVs.
Also, regarding the TBX. This is Fender's "Tone Bass Expander" control and was originally designed to work with active electronics such as used on the Clapton model with mid boost. Many folks complained of the choked tone they got on their 80's - 90' Am Std & Strat Plus models and removed them.
However, the TBX works beautifully if properly modified for passive electronics. You must remove the 82k series resistor and replace it with a 120k series/parallel resistor. About .20 from radio shack and about 3 minutes on the soldering iron. A very easy mod to do before you install it on the pickguard. This is explained at the Blue Guitar website. Here's a link to the mod.
http://www.blueguitar.org/new/schem/...nd_details.pdf
Do the mod, you'll be very pleased with the results and will have a very versatile pallet of tones.
RC
