I just picked up a Mark IV 2 days ago, and I have been running it through my Recto 2x12, and it sounds killer.
What are your settings?
Now I'm a complete newbie to the Mark IV so take this with a grain of salt, but what I have found is that if you set the bass on the preamp section too high, it gets really loose and boomy and bassy, like what you described. But if you keep the bass low on the preamp and boost it in the EQ, it remains tight. I have the bass set at 4 on the preamp section and in the eq I have the bass freq, 80hz, just below the top line and 240 on the center line. I have my lead gain at 7 pulled and my lead drive at 7 pulled. I love the tone I'm getting with the recto 2x12. Extremely tight and focused. If your expecting Recto low end, I could be wrong but I don't think your gonna get it.
Now, I have the cab lifted off the floor a bit because when I was using the recto 2x12 with my Roadking, it was too much bass when the cab was on the floor. You just couldn't dail it out.
One thing I gotta say about the Mark IV, it just floors me how good this thing sounds going straight in. Course it helps having the EQ built in. That should be a standard on all boogies including the rectos. Eq after the preamp section right before the power amp section behaves so differently from EQ prior to the preamp, you really need both!
I also wasn't expecting the clean channel to sound so good, but it sounds killer! Mind you my Roadking is a version 1, not a 2 so I don't get the lonestar cleans that you would get with a Roadster or Roadking 2.