First off Metallica had one of the greatest bassists of all time. You could definitely hear his playing. Aside from AJFA, you can definitely hear the bass in all the other albums. You don't use "stacked samples" to compete with loud guitars. The entire purpose of mixing is to level things out. When everything is in it's own frequency range and leveled properly there is no "keeping up". A huge sound is about giving things space in a mix, not volume. None of what you're saying makes sense. Do you even record at all?