Impulse responses can work for cabinets, amplifiers, things of that sort. But for basses?
The response (whether you think about it as a frequency response or as in impulse response (IR)) of a bass depends on pickup positioning, and string tuning (among other things, but these things are key). The pickup position along a string, in conjunction to the note the open string is tuned to, determines a comb filter for that string. There's the rub!
If you want to properly model a bass guitar's response, well, it's not so easy, because a bass guitar has 4, 5, or 6 responses (depending on how many strings it has) - there is not just one response. If you want to use IR's to "convert" one bass to another, the only way you can actually do that is to start with a bass that has a polyphonic pickup (so you can isolate the information that comes from each string), and an output per string. The IR program would need to run one IR per string, and then combine the result to give you the output of a different "modelled" bass. Anyone trying to sell you anything where you can take your bass's combined output, and "convert" it to the sound of another bass....is selling you snake oil. The Physics preclude that actually happening with any degree of accuracy.
