For lack of a better word, it's just hard to know how much different the "modeling" in the '51 is compared to the RH450. Since everything happening is digital, it could be vastly different despite it looking like the exact same head under the hood, even with the Tubetone and Spectracomp completely off. I think it would be fun to try and make the RH450 sound exactly like the '51, but until someone can manage to do that and give us the exact settings, I'd be a bit leary of drawing any conclusions on the '51 without actually hearing it myself.
Said another way... From what it looks like to me, they could hypothetically have a whole series of heads based on the RH450 that emulate a GK, Eden, Ampeg, or whatever else right out of the box. From that point, knob twisting would change the tone, but the changes would be based on that "modeled" voicing so that you really couldn't make one sound like the other.
Also, for the sake of clarity, I'm not saying that the rh450 is a modeling amplifier. The RH450 has one voice, with an EQ, compressor, and tube emulation circuit. It's not really trying to be a chameleon like the LINE 6 stuff for example. But there is also no doubt that much of the voicing is in the digital processing that occurs in the preamp, and that is an EASY thing for TC to change based on their goals, especially on a signature head for a high profile player like Rocco. Change the DSP parameters, load it on a ROM chip, and it's a completely different head that looks just like all the others. That's a cool thing I think, as long as the DSP is authentic and convincing, which IMO is what makes a lot of the LINE6 stuff so lame. The LINE6 gear tries to do to much and doesn't really do any of it well personally, at least not for live use. The RH450 on the other head does what it does very well.