@ JES: I don't own the FX8, but the algorithms are purported to be the same as the AxeFXII, so with that in mind...
Am I going to get the quality of pitch shifting and reverb I get from my Eventide H9?
I've not used the H9, but have owned or auditioned a lot of pitch shifters and reverbs. IMO, the reverbs are top notch, especially in High Res mode. Pitch shifting, no matter what unit is being discussed, always seems to be a very individual thing in terms of preference. I've owned the TC Electronic G-Force, Digitech Whammy, and EHX Pitch Fork. IMO the pitch shifting is excellent by comparison, especially if you want shifting with minimal shifted tone coloration, which is something all of the aforementioned units suffer from IMO. Personally, I've had to purposely filter/distort the pitch shifted tone(s) in order to make them less pristine sounding so that they'll better emulate the pitch shifted tones I'd become accustomed to. I don't view this as a bad thing - the accuracy of the tone - but rather something to be aware of. IMO it is better to have this and not need it (or need to alter it) than vice versa.
Are the overdrives and fuzzes (with EQs, of course) strong enough to replace a BB pre, OFD, Brassmeister, Bassdrive, Red Dragon, Wooly Mammoth and/or other bass pedals?
The answer: IMO maybe. If you're a overdrive/distortion/fuzz junkie and you know the circuitry that makes a pedal yield it's own specific flavor of distortion, then chances are you'll be able to edit the Drive parameters to your liking using an existing Drive Type as a starting point. And because you can do parallel routing, this makes blending the wet/dry mix a lot less hassle. True, there aren't any dedicated Bass guitar overdrive boxes modeled, but that doesn't mean they won't sound good - or cannot be made to sound good - on bass guitar. You might just need to tweak some of the parameters (low/hi pass filters, EQ, etc.) to get them closer to where you want them. I've used the Rat drive many times and find it to be a relatively easy adjustment for bass. Realistically, it is difficult for a multi-fx device to absolutely nail down every flavor of "popular" overdrive/fuzz pedal. The list is relatively comprehensive IMO, but I'm not the biggest bass distortion junkie out there either. YMMV, but as with everything Fractal, the quality of these drive effects is very good IMO.
How are the filters compared to Iron Ether or Moog offerings? Can you do synth-like filter sweeps that have decent low end?
IMO, yes. And this is where I wish the synth block would be brought into the FX8. The filters are very good, but not designed "as is" to be akin to a "synth filter". You would have to attach a controller (Envelope/Pitch/ADSR/Sequencer) in order for the Filter to behave this way in an "automated" form, or, use an expression pedal for manual control. Filter with drives can be made to sound plenty fat and beefy. Adding Synth would have put this over the top. You'll have to go AxeFxII for that though.
I've used them often, and they sound spot on to me.
Can you use multiple instances of the looper to have A/B parts of a piece?
One instance of Looper, to my knowledge.
Are there things it does for bass that no other effects setup does?
Perhaps. I wouldn't assume that the stock preset library is going to necessarily scratch that itch, but I will say that the modifier/controller aspects of the Fractal devices are absolutely unrivaled in any hardware device I've ever encountered. Thus, if you understand how control modifiers work and how to implement them creatively, then YES, you can make it do for YOU some very unique things that no other single processor can do, other than another Fractal product like the AxeFx. For example, I attach a Pitch modifier to the Input Level and Feedback of a Delay that is in parallel to my dry signal, so that when I play high notes, the sound is drenched in delay, but when I play low notes, it automatically backs off the level going into the delay and reduces the feedback, drying up the tone. This is just one very simple example.
And finally, how easy is it to tweak your presets when you get to practice or a gig and realize your levels are off?
Couldn't address this, but from what I can tell, it is designed to be fairly quick to tweak live. YMMV.
The FX8 is a stripped down version of the AxeFxII, in pedal form, and aimed at those who don't want or need amp/cabinet modeling, and would use the FX8 as a front end virtual pedal board, or in conjunction with their existing amp's effects loop (or 4 cable method) with emphasis on there being zero "tone suck" altering the fundamental tone of the instrument.
Again, by design, has comparative limitations vs. the AxeFxII. If you want as much "whizz bang" as you can get in terms of effects & processing, IMO the AxeFXII is the way to go. The FX8 is still a very nice piece, though, no doubt. Fractal has a no questions 15 day trial period. Hope this helps.