I can't comment on the Aguilar having only played through one on several occasions (other than to say it sounds awesome!) but I can give you some insight on the other two.
Main difference between the Alembic and the Dem is that the Alembic will add a warm, slightly scooped old school tube tone to the sound of your bass, whereas the Demeter is going to be more transparent and hi-fi. If you dig the sound of your bass and don't want your preamp to add much to it, the Demeter is probably the way to go. But if you're looking to tame the beast a little bit and warm it up at the same time, the Alembic might make more sense.
Personally I like how all of my instruments sound through my Alembic, as it adds a nice studio warmth without losing any of their built-in flavor. However, you may still need to add an external EQ to get more versatility out of it.
At any rate, all three are great options - you just need to decide what kind of role you want them to play in your tone.
__________________ Nearsighted monitor engineer: "What the hell is an Anemic F-1X?'"