Another thing I am missing here is why you would want to do this. It seems like categorically the worst option. Even if you don’t want to buy a big bass amp and cabs, instead of this you can get 2000 watt powered PA speakers for $250 each. Just run your tone-coloring preamp into a couple of those instead. Mackie makes a 1300W 15" one for $350:
But really, just getting a less expensive, modern version of something like the 800RB you used to have seems like the best option to me 
Even used G-K modern amp heads aren’t too bad. Used MB500’s run $300-400. And G-K is on the spendy side of course.