There is no best bass for metal. A car subwoofer is not a bass speaker. Few speaker swaps are actually an upgrade over stock. Loud, little, low; you can only have two. Guitar amps work fine for bass, but guitar speakers usually don't. A 15" speaker does not inherently offer more low end than a 10", which doesn't inherently have more punch than a 15". If you need more volume: adding more speakers, increasing the surface area by using larger speakers, or using more sensitive speakers are typically more effective ways to accomplish that goal than getting a more powerful amp. Bass guitar lives in the midrange, don't eq it out. Cabinet watt ratings only tell you an electrical maximum, you can damage speakers mechanically with far less power. Mixing different speaker designs (like differing driver sizes and/or configurations; or ported with sealed) that receive the same full range signal usually turns out bad not good for your overall sound or gear health.
That covers some popular TB ground.
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