xhypo2dude... are you aiming at getting a particular sound from lets say a j-rock song or a band who's bass player's tone you admire? In that case, maybe the best is to get an equalizer of some sort... An equalizer pedal will probably be the most practical, though you can go overboard with a 31-band rackmount model you can carry around in a rackcase haha... Just kidding. The eq section found on some amps are excellent, but not all the time you get this flexibility (dependant on the amp, unless you bring your own amp around for gigs or you play only at one venue eg church); the only consistent way then is to get your own eq device. The amp of course adds its own tone even with the eq set to flat in terms of speaker driver used, tube or solid state etc blah blah, but for bigger venues (or during recording) usually the bass will be DI-ed out to the mixing board before your amp, so your tone will still be your tone to the audience. Unless the soundman itchy hand change the tone for you on his board (sometimes they do that if your tone sucks in their opinion haha... which is true, good soundmen make you sound good. Eq control on your hands on the other hand, makes you sound good if the soundman is the sit one corner and drink coke during a show kinda guy). The amp is only what you hear onstage for monitoring.
lowdown: simply put what an Equaliser will do is change your tone. Eg your bass is boomy but not very bright. Using an Eq you can cut down the bass and boost the treble frequencies to even out the tone. You can also use it to try to get as close as possible to the tone of another bass player you admire, of course minus effects eg if that player uses chorus or overdrive then that's extra. But if that bass player plays clean then you can get quite close, again dependant on the level of control the eq you get offers; Boss GE-7 only 7-bands of tweaking, Boss EQ-20 has 10-bands if I'm not mistaken which allows you to make finer adjustments. Of course there is another thing called compression that makes the bass sound full on recordings, but proper eq can get your tone close enough for most live applications.
Maybe the common 7-band graphic EQ by boss is sufficient, or you may want to go with more tweakability with the Boss EQ-20 with more frequency centers.
Basically what you do is listen to your tone, compare to the other player's tone and try to note the difference. Is your bass tone too bright compared to his? then experiment cutting a few sliders in the higher frequency (one or more) to get the same effect etc.
There is another eq besides graphic eq: its the parametric eq. But I can't seem to find one in a pedal form around anywhere... Try out the graphic eq, it should be the thing you're looking for.
(P.S. another option is to upgrade the electronics in your bass to something which offers more control, but that's the more difficult way somehow... With external effects / eq controls you can always 'undo' easily. By selling away the effect pedal haha)
