I am currently going through a very serious case of GAAS, and have a RoadKing S1 combo, Mark IVA, LSS, 5:25, StudioPre, and I have a VHT Sig:X on the way.
The point is, I am trying to do side by side comparisons of as many amps as possible and so far the Mark IV is still the all around winner, the one I would not part with under any circumstances. Don' get me wrong I really like all the amps for the variation they provide, but this is my third Mark IV and this one is not going anywhere.
I have done this with my guitars too, had about 20 a while ago and I am now down to 5, which I am really happy with and not gassing for guitars at all. Would still like to get down to 3.
My recommendation is keep the Mark IV if you possibly can. Go out on a limb if you have to to swing the Roadster, they keep their value so it unlikely you would loose anything by doing an A/B comparison.
Good luck with your hunt for "the amp"!