After having Rocket 44 for about a year I made the change to Rocket 88. There was an improvement in that it did provide a blacker background to music. I tend to believe this is primarily due to the 88 including the Black DBS devices more than anything else. Then, after a year with the 88 I made the plunge to William Tell ZERO. Oh my, that was a revelation of improvement. I am now hearing musical information in a more distinct manner than ever before, particularly in nuances of detail that seems to trail off forever. Instrumentation and vocals have never sounded so distinct and precise in my system. I believe these differences are largely due to having the Carbon DBS devices, the geometry which provides zero characteristic impedance, and the multi-layer carbon-based noise dissipation system. The terminations also use a higher grade copper than the Rocket series. Both series use the same hanging silver plating, though. In other words, the step-up technologies in this series of cable definitely delivers on it's promise. It's equal to the other best audio investment I have made in my present system. That other investment was having my BAT VK-33 preamp upgraded to the VK-33SE. The difference in imaging was quite stunning. Then I added the William Tell ZERO and fell off of my chair but only after my jaw hit the floor. These are exaggerations, of course. My jaw is not that long and I really didn't fall out of my chair. But sonically, these speaker cables are also quite stunning, no two ways about it. I'll make an analogy for you. Imagine looking through a window of exceptionally clear glass at a beautiful mountain scenery of Fall colors. The glass is so clear that you would think it wasn't even there. And then you open the window and realize, oh my, this is even better.
You asked why pass on the Robin Hood ZERO and go to William Tell ZERO. My answer is that I felt better about having more wire. The Robin Hood Zero consists of 4 solid PSC+ copper wire at 15 gauge. The William Tell Zero consists of 6 solid PSC+ copper wire at 13 gauge. Technologies, otherwise, are the same between these two cables. Sure, it's more money for one over the other. But I didn't look at it that way. I saw it as that is the gauge of wire I want for my speakers and that is the technology I want for my speaker cable. I couldn't be more happy.
Best to you. I hope this explanation helped and that you end up with a satisfactory decision.
Note; I also use Tornado power cables and the Niagara 3000. For signal cables I use Audioquest Earth. The synergy of cabling from Audioquest is a wonderful way of connecting your entire system.