Just a guess, but by "half stack" and "stack" i assume you mean one or two 4x10 cabs or something similar. FWIW, terms like stack and half stack are pretty ambiguous in the world of bass, whereas in the world of guitar everyone understands you mean 1 or 2 Marshall (or clone) 4x12" cabs.
Just how loud is loud? If you want to get really really loud small may not cut it. Come to think of it, who's really going to stay to watch your band at that volume? Consider bringing it down a notch or 2.
For cutting through in a metal setting all that you may need is some EQ. Go put on a pantera/metal CD. The guitars have scooped mids and boosted highs and lows. To cut through, the bass has the lows cut (or at least not boosted), and often has a boost somewhere in the mids. Just leaving your lows flat or even cutting them a bit will allow you to turn up more. You may not like the resulting tone when you play by yourself, but when the entire band plays it works for metal.
If you do have to get a new rig, cabinets are going to give the most bang for the buck volume wise. Cabinets with high sensitivity ratings will be be much louder, and no matter how many watts you put into a cab with a low sensitivity rating, it's not going to cut it. You'll also get more dispersion stacking a bunch of smaller cabs vertically than from bigger cabs on the floor. If the cabs alone don't solve the issue take a look at some high wattage heads and/or poweramps. Some are louder than others, old trace elliot heads and the aguilar 750 are
much louder IME than similarly rated heads.
as far as "grit" unless you go with an all tube head like an old SVT, that tone will be a function of your preamp, EQ settings, and outboard overdrive/fuzz type units. Likewise some basses have more of "that sound" and even something as simple as brand new stainless steel strings with a bit of high mid bump can be all the Dr. ordered.