Well, here's my tonal assessment of all the heads I mentioned previously. I'll keep it to tone as the power delivery is quite varied between each unit. These are my personal feelings, you might have a different feeling of them.
GK MB2-500 (owned)
Big, big, big, big, big bottom end. To get close to the flat tone of any of the other heads, this unit requires considerably cutting of the low end. Outside of the low end bloom, on all 3 of the units I owned, which ran from the initial release, up to A06, it sounded basically like any of the modern RB heads from GK. The upper mids had a slightly different character that I didn't like quite as much, but the difference was absolutely minimal. Huge bottom, not a ton of lower mids, aggressive, but not overly aggressive upper mids and a top end that was very high. This was a very usable head once you reeled in the low end, but never left me feeling like it was special in any way like the other GK heads I've owned (2001RB, 1001RB) and absolutely loved.
Markbass LMTube (owned)
I would put this into the "hi-fi" category of amps because of it's wide frequency response, but it only barely makes it into this category. It's wide, but not overly so. If you're looking for something that's a compromise between an old-school voicing and a modern, wide, hi-fi one, this is a great choice. As KJung has pointed out, the low-mid EQ center is a bit high and doesn't really help to fill out the usable bottom end of a lot of cabs. I really agree with this. If you have a cab with tons of low-mids, the Ampeg 410HLF springs to mind, this probably wouldn't be an issue. Something like an Epifani UL series cab and you're left sounding very thin in the mix. Great bottom end, weak lower mids, good, but not great upper mids (can sound a little harsh), and what feels like a top end that could be voiced just a little higher to add more sparkle and sheen than what's available on tap.
Tecamp Puma1000 (currently own)
My personal favorite. Extremely wide voicing that goes subterranean in its low end and stratospheric in it's top end. The low mids are voiced to really thicken things up and the upper mids are polite and very Marcus Miller'esque. That all sounds great, except that you're not left with much else. I would never use this head in a rock or country setting and be blown away. For pop, jazz, funk, etc. I absolutely love it, but it's relegated to those contexts and little else for me. When I use it in another setting, I always put something in front of it to reign in it's wide tone and add a different midrange character, specifically to the upper mids.
TC Electronics RH450 (used, but never owned)
I'm the black sheep on this one. Hated it. Absolutely hated it. The lows were big, but not overly so. The lower mids were warm and filled things out nicely. The highs were slightly muted giving a greaty old-school character.... But the upper mids... Oh the upper mids... Yuck. No amount of semi-parametric EQ tweaking could get the upper mids to sound anything other than overly aggressive, ganky, nasal, etc. I tried as hard as I could. A lot of people have great results with this head, but I simply did not. I messed with the Spectracomp, the tube drive (can't remember the name of the OD on this unit), the EQ, the gain... Basically everything and was left with horrid upper mids that simply did not work. I went as far as to try the head through almost all the cabs at BassNW and could not find a combination that I liked. I really wanted to like this head, but it just didn't do it for me.
Genz Benz Shuttle 9.0 (used, but never owned)
Similar in its hi-fi nature to the Tecamp Puma1000. This is my favorite Genz Benz head to date. I've owned a GBE750 and was not impressed in the least with the Genz Benz tone. I tried the 12.0 and found myself thinking it was basically the same as my previous GBE750. The 9.0 surprised me in that it was much better. It's a very hi-fi sounding head. Compared to the Puma1000, it goes just about as high, but not nearly as low. I find myself EQ'ing in bottom end on most Genz Benz heads to fill out a mix, and the 9.0 is no different, but by far the best I've tried. The lower mids sound good, the upper mids sound good, the top end sounds good enough, but can get a bit harsh at times. The low end is where this head lacks to my ear. It just seems thin. Again, this is, to me, the best offering Genz has in the low end department, but there's always the feeling that it's either too thin, or too bright, sometimes both when I'm using one of these heads without drastic EQ'ing. That's not too big of a deal for me as I do believe that EQ knobs are meant to be turned. The problem is that I find myself EQ'ing into a corner to get a starting tone that works and am not left with much room to maneuver in tricky rooms.
Mesa Walkabout (used, but never owned (previously owned an M-Pulse 600))
Tons of booty, tons of mids, muted top end, lots of "give" and "sag" that makes you feel at times like you're playing a tube amp and an overall pleasing tone right from the start. This is a very easy amp to make sound good. I frequently find these amps, and the M-Pulse models, to sound a bit mid-heavy in a mix, but not unpleasantly so. There's a character to the midrange of these amps that the other amps in my write-up simply don't have. That said, it can definitely sit too far forward in a mix. It's not nasal sounding midrange though, so you may or may not feel the same way. It's a really hard thing to put my finger (ears?) on and describe accurately. It's like there's too much of a good thing going on that you dislike, but also don't mind. Talk about cryptic huh?