Here's some stuff on Yamaha basses, for all those lost in the model number/letter jungle.
First of all, TRB series. Yamaha's high end series, directed at sort of sophisticated yet progressive jazz(ish) player. John Patitucci is a perfect example of this kind of a player, he has his signature TRB6JP. The TRB II's are improved version(improvement mainly on electronics), although some claim that the first series has better tone.
There have been exotic wood and piezo equipped TRBs around, these costing well over 1000 bucks.
Then there's BB series. For players seeking (semi-)traditional axes, BB's look great in blues/rock band. They have P-ish appearance both in looks and in tone. Nathan East is their mannequin for these basses. There are quite many types of BB's around, from old BB300 etc to more recent BBG5S. Interesting tho, they are now going back to numbering the new BB's, first of these being the new BB1500A.
Short description of letter-marked basses: BBN4 - the basic BB, with all passive stuff. Very inexpensive. BBG4 is similar to BBN4, but has a foto-flame top.
BBG4S - BBN4 upgraded with active 2-band EQ. BBG4A is similar but with foto-flame top.
BBN5A - now this is getting somewhere. This is a bit of a downgrade from Nathan East Signature, but still houses 3-band EQ, similar contours to BBNEAST(the signature model), real top, etc etc. Price of BBN5A would be somewhere around 700-1000 bucks IIRC.
All new lettered BBs sold now are pretty much the second or third (II or III) versions, the main difference being that earlier versions have jazz-style pups, whereas later BBs have large humbuckers.
The RBX series seems to be pretty similar to BB series, the difference being Yamaha's going for young players this time. So the RBX's are shaped more radically and their electronics, I would assume, are designed for rock/metal, but the parts remain pretty similar. For example both BBs and RBXs have 2-band EQ on their better models, similar humbuckers, etc. However, there's no 'semi-highend' model as BBN5A below signature(which belongs to Dream Theater's John Myung), but to accomodate this the RBX6JM is priced pretty low.
I'm not connected to Yamaha in any ways but if you have something to ask about different models, I know pretty much about them.