I'd make the Keytarist play Basslines. If anyone can, a keyboardist can. There are plenty of perfectly good synthesizer bass sounds.
I'd let the Bassist continue acting like a lead. Then you come in with your guitar (what's the best guitar for metal anyways?) and embarrass him off the stage with sweep-pickings and two-hand tapping. Also, use your whammy bar, because basses usually don't have those. That's the way it is. By the way, if the bassist is getting louder than the guitar, then you better get yourself another stack.
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I've seen the best bass for metal. Pictures have been floating around on this forum, but it's a best kept secret to outsiders. I stumbled upon the pictures because, as noted, it called out for me. Just to admire though.