Most metal wont' go low enough for port tuning to make a heck of alot of difference. Spend your time worrying about your front speakers.. I had some very nice pro audio mids and horns in my last car and people loved rock and metal in my car, even with subs off.. Subs just added that last 10%..
Also, for metal you want to tune fairly low.. Alot of this forum tends to preach tune high for metal. Their idea is that since metal doesn't go low, you should tune higher, but that has 2 main issues. First, most of the subwoofer duty for metal music is right near the usual crossover points, between 60-80hz, with lower drums being 45ish.. Tuning anywhere between 30-40 isn't going to make much of a difference between what's going on most of the higher stuff... It does make the 45hz stuff louder, but for metal "loud" bass isnt' really what you want.. You want loud bass that keeps up with the music and still sounds like the band playing it... Secondly and most importantly tho, the higher you tune the higher your group delay and the higher in frequency it occurs. Tuning near 40 pushes your delay right up and into your metal music making it sound sloppy and slow..
If you want good bass for metal, do 3 thing, in this order
1.Get good mid basses (deaden the doors, or use your kickpanels, big speakers, 6.5 minimum, high effeciency and power)
2.Get a sub that can actaully play up to 80hz+ and let it do it's thing
3.Either use it sealed, or if you do listen to some rap, do a small to medium sized ported box tuned low 28-32hz, depending on taste.