I know you've played a few different Mesas and are also at least relatively impressed with the Mark-IV, and I'll also state for the record that I'm not personally a MASSIVE Rectifier fan...
...but, honestly, the "buzz-box" you're referring to isn't because the amp sucks, it's because a large chunk of Rectifier owners, and about 90% of the GC amp-posers, are complete morons.
Simply put, it's a very complex amp, and when you consider there are three distinct preamp modes per channel that all interact with the tone controls very differently, you CANNOT set Modern mode of Channel 3 (the "Recto" sound) like you would a Marshall, and not expect it to not sound like ass.
Modern mode has a substantial gain increase over Vintage (which itself is juiced compared to Raw), and significantly more presence and a MUCH more robust bass. The lower midrange is also a bit more scooped than it is on the first two modes. As such, you need to be careful with how you set the rest of the controls - I've found that while you can run the gain fairly high on Vintage, and safely max it out on Raw, 6 is really about the "useable" cutoff on Modern if you want any articulation. Likewise, unless you WANT that harmonic edginess and sizzle, you need to be careful with presence and treble. I forgot what I found that worked for me, but I think it was setting the presence very low and the treble in the halfway range. The bass, due to the extreme amount of low end already in the signal, needs to be set VERY low or it'll turn to mud - I mentioned below 3 earlier on, but I believe a buddy of mine got great results with a Triple on his last album with the bass on 0. Likewise, the midrange needs to come up past the usual 1-3 range most metal guys scoop to - it's a scooped enough midrange as it is, so you really don't want to scoop further. I want to say I was getting good results in the 6-7 range...
And this of course all changes when you start playing with the back panel Bold/Spongy and Solid State/Tube Rectification controls. :lol:
Anyway, set correctly a Recto isn't a wall of buzz - it's a brutal, articulate tone that'll cut through a mix with no problems. On my PWH, the neck pickup through 3 Modern set intelligently just BEGGED for fast Petrucci-style runs, while the bridge was perfect for riffing.
Anyway, moral of the story is that because there's so much variation between each of the "distortion" modes, there are areas of the EQ range that are just not particularly appealing in each mode, and the sad reality is the gain cranked, 10-0-10 EQ that most GC metal heads default to is one of 'em for channel 3.