i play metal. and while rack effects arent my thing, i DO know that bassists dont use effects much in metal. unless, that it, youre playing stoner metal. which then all you really need is a fuzz and whatever else you throw in there. i play with 2 guitarists, which makes it infinitely easier to be buried in the mix than if youre playing with just 1. all you need for a good rack setup is a tuner, a compressor, and a preamp. a tuner, obviously, is, well, a tuner. if you dont know what a tuner is, theres not much help for you. a compressor, in a nutshell, prevents volume spikes. lets say that youre playing a fast, percussive, metal bassline. suddenly, you hook your finger under the string to far and that note sticks out MUCH louder than the others. well, rather than having babababa
BAbababa, itll be more like babababa
babababa. wont even it completely, but it will help prevent a huge spike. which, trust me, get in the way sometimes
lastly, lets get to a preamp. a preamp is something that alters (for better or for worse) your clean bass tone, mainly via better eq adjustment. alot of metal bassists enjoy the sound a tube-based amplifier gets. but since not all of us can afford a nice tube amplifier, some will opt for a preamp or head that emulates that sound. ampeg is partially responsible for the popularity of tube amps among bass players, and their non-tube heads are often made with a tube emulation circuit. they also have the svp-pro preamp, which uses 5 (correct me if im wrong) tubes, so you can have that natural tube tone. tech 21 also makes a rack preamp, but it is a tube emulator. your best bet for a relatively low-cost tube emulating preamp will probably be found in a pedal rather than a rack effect. tech 21 has the bass driver di (bddi), para driver di (pddi), and vt bass pedals. the first 2 have a direct xlr output, which enables you to plug in directly to the soundboard without using an amp if youre playing a gig. the para driver has sweepable mids, and a mid control to begin with. the bass driver can (from what ive heard) get a little more drive than the para driver, but ive never used the latter. the tech 21 vt bass gets alot of good reviews around here. it can be used as an eq control, and it is also capable of a grindy, tubey overdrive as well. im sure theres people here who know what theyre talking about better than i do, but this should get you pointed in the right direction. effects wont do a thing for you if you want to cut through in a metal band
lastly, read this
Demystifying Metal styles, how to make fingerstyle playing heard in Metal