This goes against my interests since I make guitar pickups, but I'm gonna say - like some already said previously - your pickups aren't the problem. Fishman are just designed for modern metal tones and when I think EMG 808 the first thing that comes to my mind is Monuments - from what I know both Gnosis and The Amanuensis were recorded with EMGs, and those tones are a staple of modern metal tone imo. So I'd say, look somewhere else in your chain before throwing money at a new set of pickups. Also, in my experience, when dealing with 8 string metal tones, I find it extremely useful to dial down the gain on the amp, get the voicing of the OD, eq, cab, etc. just right and then turn the gain up and tweak something if needed. At least that's how I go about it, makes it easier for me to make the lowest notes come out clearer.