Somehow never saw this, you've probably done something by now... but... and this will include personal remarks, there I said it...
I finally learned to not like Markbass, but it takes me a while to know my ear with an amp. I find it too hi-fi, too pasteurized- when I hear reggae bass players using it I miss something significant from their tone. Descriptive word: "Bloodless".
Fender... more old-school, more essential. Less thumpy than Ampeg, but also less dangerous sounding. I'd say it's a very useable sound, I could probably go anywhere with it. A little genetic, but that's good if you're an itinerary player and seek a reliable working sound.
GK. That active eq and the very engineered power section, which hasn't changed much all these years, is a pretty deadly combination. It's definitely a flavor, but still an able and adaptable machine.
Slammin', crunchy, internal organ-rearranging rock tones and power are in that little box.
Sweet, articulate snazzy-jazzy tones are a-poppin'.
One of my reggae band mates commented that GK's not voiced well for reggae, but I don't agree. Though it tends to be not so deeply voiced as my old Trace head, it's not significant, as in most situations that kind of eq is lesd than dedirable anyway. Muddies up the FOH mix... the eq is well able to give you your dub fix.
GK is killer gear, whatever model you decide on.