Hi Bri,
You're right, the GK MB210 doesn't have a detatchable head, but do you need it to be removable? It's an advantage if you're doing sessions or whatever, so you can keep "your sound", but the MB210 only weighs 16kg in total, which, believe me, is an incredible advantage when it comes to getting your gear in and out of venues. My bass goes over my shoulder (straps, y'see..) case for accessories in one hand, amp in the other! Simples! Gear moved in one hit - definitely good when it's raining...
I haven't tried the Promethean, the MarkBass is out of my price range and the Harley Benton is a little[i] too[/i] cheap (I'm such a snob...) so I can't really comment on those amps, but the GK does what I want it to. I use a passive jazz bass, played fingerstyle it is incredibly warm sounding, played with a pick, it sounds hard, with plenty of attack. The bottom end is tremendous for a 2 x 10, so much so that I intend to buy a stand for it as a lot of the low end goes through the floor. So, to recap - 16kg, 300w, £480. You pay yer money, you makes yer choice. I love it.