I haven't played through one of the new ones so I don't know how close it is to an OG 1001rb. If anyone has done a side by side, I'd love to get their take.
I haven't played through the RB-Legacy, but I did own an 800RB, 700RB-II, and my Legacy 800 at the same time, and did multiple round-robin comparisons.
The 700RB-II and 800RB sound most similar, but the 700 goes lower and higher, and I preferred how the 700's Contour and Presence were knobs rather than an all-or-nothing switches.
With all knobs at noon, the Legacy has a lot more midrange. Legacy lows at flat are comparable to the 800 and less than the 700.
The overdrive section of the Legacy is useful to get the "rails" tonality - but your results are closely related to how much you trim back the input: GK recommends running trim all the way up, then only reducing it enough to avoid clipping - but that approach gave me too much breakup in the overdrive section.
Starting with Trim at noon and experimenting gave me better results.
Comments from NAMM suggest the RB-Legacy might have a thicker starting tone than the Class-D Legacy - but I'd have to demo one in person to see if the tonal gains outweighed (pun intended) the size increase.
But I'm pretty darn happy with my Class D Legacy - and I rarely play gigs where I could be loud enough to push an RB-Legacy "into the rails".
So as much as I love that these new amps exist, I'm unlikely to buy one.