Before we go blindly recommending amps:
What are you going for tonally, and what is it you don't like about the Mark IV? What, specifically, is the Mark IV not doing for you?
If I may ask, what are your Mark IV settings? The Mark IV's controls are sort of counterintuitive compared to a more "normal" amp. It's very mid-heavy, to the point where you can't really dial out mids without the graphic EQ, and the amp's bass control adds more mud than anything past about three or four on the dial. And it's very dark without turning the treble up. Despite all these seeming "issues", the Mark IV is actually capable of a wide variety of tones due to the graphic EQ and power stage options.
I used the following settings with an EMG-loaded Jackson tuned to B or A for death metal with my Mark IV. With a boost pedal and the EMGs, it was the tightest thing going next to possibly a VHT.
Lead Channel, Full Power, Simul-class (Mesa EL34s and NOS GE 6L6GCs), Triode.
Lead gain- 8, Fat Pulled
Bass- 3
Mids- 4
Treble- 8
Lead drive- 6
Presence- varies, depending on room
Graphic EQ- Boost 80hz, cut 240hz, leave 800hz flat, boost 2200hz slightly, boost 6600hz.
Boost settings (I used a Boss SD-1): Level 5:00, tone 1:00, Gain 7:00
...and a cab with Vintage 30s.