That's really not the question here - it's about what you do, not what you use, all I can tell you will be along the lines of 'EQ this, compress that, be careful with that level' and I'd recommend strongly against viewing a plugin as anything more than a tool to improve something once a clear plan of action (or a lot of alcohol) is present. I'll loosely describe what I'd do, though.
First, I'd try to see if the lack of definition/clarity came from a very low-oriented and high-lacking sound - bit of a boost at 1.5kHz, another boost around 750Hz, cut around 50Hz, perhaps tinkering with a shelf around 3kHz; not trying to get the mix sound with that, I'd use that kind of thing to determine whether or not the sound is usable at all, as it's entirely possible that something was done wrong earlier in the chain and it's better to just rerecord something if you know it can be done better rather than try to bandage a missing head. If you can start to hear definite notes coming out (when it's mixed! Wouldn't dare to judge this stuff solo, at least have drums and guide guitars, close to what you'd want in a final mix if the bass is your problem here) then your bass sound is just a shy little flower, waiting to be coaxed out to see sunshine and happiness and cute cuddly bunnies hopping through the fields; then again, that doesn't always happen, so if after a good chunk of time playing with high boosts and low cuts you still feel like you're trying to sculpt mud, throw it out like the fat friend you had to pork for a buddy and try something new.
If you rerecord, monitor things very carefully - get a bright sound with cotton in your ears, don't make the same mistake twice unless you can clearly pin the blame on the aliens for one, if not both, of them, and maybe double it at the source and try to get some brightness out of a well-miced bass amp. Go for fucking Rush out of the amp if you have to, if you're going Iced Earth pull a Schaffer and completely overdo everything on that poor little Ampeg if you think you can blend it with the DI in the mix.
If you can start to pull intelligible lines out of the track, and it doesn't turn to complete shit when you've done so, try and figure out how little you can get away with and still make sense in the mix. Don't worry about level just yet, if you can hear notes from the bass and the rest of the mix is still all there you've won... probably about three-eighths of the battle. If you can pull something that works out of the track, without resorting to 18db shelves and enough notches to make your screen look like it tried to invade Normandy, hit a compressor if it feels like the level bouncing is distracting from the rest of the mix. Some bass players are like jackhammers, others are like jumping beans on PCP - you should know what to do from here, use a higher ratio if the notes are jumping all over the place and you can't make one note sit without the two around it disappear and use a lower ratio if the bassist is solid and just needs a little taming. I'd start off with a fairly quick attack and release around two-fifths of a second to see if it's possible to just hug the thing into submission, but spend at least five minutes on each knob you have available if anything makes you the slightest bit uneasy, and if you start to hear 'pumping' back off before it's too late.
Hopefully you can put up clips at some point, that makes all of this a lot easier.
Jeff
