The dark tones don't come from amp. It comes from wood (and pickups). You should search for bass with darker woods. Wenge for example. I played some cort, 500$ price, has wenge neck with rosewood fingerboard, it had the darkest tone I've ever heard, deep to. But I like my tone clear and round.
Google it up, dark woods = darker tone. Why do you think maple fingerboard is making clearer sound than rosewood? You need more dark woods to get more darker tone. I know that warwick makes basses with wenge neck and wenge fingerboard, or something like that. Look that up.
And again, amp had NOTHING to do with the tone. It is just (as the name says) an amplifier. It amplifies sounds.
I'm fan of flat eq and even eq bypass, and I can hear my tone on almost any amp. Whether i play without or with amp, I get same tone. So the biggest factor is wood, after that pickups and strings, I know that bartolini has some darker bassier pickups,look that up too. And I don't like them just because of that

good luck