I can't believe no one has said this yet and if I missed it and am repeating someone, I apologize.
First of all, I own a MB150 combo. I also have a EA Wizzy Combo and in past have owned and RB400 head, with a few different cabs (including the MBX), an Eden head, Ashdown and other stuff that I may not even remember at the moment. I used to double on gigs (EBG and DB), but now it's 1 EBG gig out of 99 DB gigs and no switching back and forth on the same gig.
Now, all that out of the way. The MB150 is made for double bass and works quite well, although it does have some limitations and some people don't like them for personal sonic reasons. There are alternatives now in that weight/power/function range that didn't used to exist and it's valid to "prefer" the GK vs something else or not.
There's nothing wrong with plugging an elecric bass into a MB150 and I do it more often than not. I would not suggest this for playing electric bass with a band on stage or outdoors or anything bigger than say a coffee house gig that required climbing several flights of stairs. It's just not what it's for, no matter what ext cabinet you use.
I don't think the other GK heads are voiced or impediance matched as well for doublebass. So in other words, don't let the fact that a lot of DB'ists use the GK MB150 goad you into thinking the GK700whatever will be any better than another brand would be.
For people who truely double, the Mark Bass and Euphonic Audio are generally better respected and with the money you're talking about spending, you could get a good set up there.
Take it for what it's worth, I hope it helps.