Since you haven't found anyone who can directly compare the two amps I'll give you some probably not very relevant impressions based on my own experience.
I owned the Rogue Cronus, not the Magnum, and now have the Sli-80 F1, an upgraded version of the standard Sli-80. I didn't own them at the same time, though, and used the Cronus with DeVore Fidelity 8s while I've used the Sli-80 with Super 8s.
Both are great amps and my reasons for selling the Cronus may not be applicable in your situation. I appreciated the dynamic sound the Cronus produced but ultimately preferred the lower-powered triode output of an AES/Cary Superamp I had at the same time.
I also had a lot of trouble with microphonics in the small signal tubes, it felt as if the tubes were being driven much harder in the Cronus to get that much more power from a similar tube configuration.
Being able to bias tubes individually on the Cronus rather than by channel as is done with the Sli-80 would be an advantage with unmatched tubes but getting to the bias pots was a little inconvenient. That has nothing to do with the sound, I know, but it was a minor irritant to me.
I'm absolutely in love with the Sli-80 F1, the build quality, the way it works and looks, and especially the sound. The combination of it with the DeVore Super 8s and the Simaudio 300D DAC means I'm not looking around for something new for the first time in a long time.
I've had great service from both Rogue and Cary.
So, sorry, this probably isn't relevant to your situation and you'll definitely want to compare the two in your system if that's possible. They're going to sound different but I wouldn't try to predict whether that would be an improvement with your speakers.
Good luck.