What he said about the guitars not leaving any room for the bass is a really valid point. I've been in bands before with similar issues, so I just turned my mids up. There' a band called Spiral Architect (or it might be Spastic Ink, I forget), and their bass player plays a fretless with lots of mids, underneath guitars with lots of low-end and gain. It works beautifully, and there's almost never a moment when he can't be heard. It fits the music too, because it's a bit avante-garde.
The point is that you have to think outside of the box, and the box in question has been constructed based on the guitar, not the bass. We're told to play the same notes, only an octave lower, and make our tone diametric to theirs (which often doesn't work in heavy music, because guitar tone is frequently counter-intuitive to the concept of being the instrument in the higher register). This gets worse when down-tuning is involved, especially with 7-string guitars and 5-string basses.
In a situation with the aforementioned additional lower strings, plus down-tuning, I think ghosting the guitar is sonic suicide. YOU WILL NOT BE HEARD IN THE MIX. Some bassists start slapping, and generate high-end click that cuts through, but fail to notice the tonally contextual correlation that results. In "Jambi" by Tool, the bass plays a mini-solo while the drums and guitar maintain the rhythm. The rhythm doesn't suffer because they're in drop-D, and Adam's guitar tone has plenty of bass. This same idea of the guitar playing low and the bass playing high, while counter-intuitive, is the best way to resolve the tonal balance. It beats the hell out of hours of knob-fiddling for hours, and is more reliable because tone changes with different venues. That extra low-end that you dialed in to sound heavier than the guitarist can turn into a boomy mess, in certain venues.
Regarding Fieldy, I'm not a huge fan, but you can't impeach the success of a man who has more threads about him on a musician's website than almost any other bass player, even if they are pejorative. His tone works too, because he's playing with drop-tuned 7-string guitars and is often audible. I would never encourage any budding bass player to use him as a model for development, though. I also find it ironic that his signature Ibanez bass has a three-band EQ (including mid-range), and a frequency control for that mid-range. As if he'd EVER use it.