Rogue Sphinx....good match with Merlin TSM?

You’d be going from one entry-level class D module (ICEPower) to another (Hypex UcD). You’re also going from a ECC99 tube front-end to 12AU7. Frankly, ECC99 / 12BH7 is the better tube.

I think in net, this would be a sideways move. Maybe you like the Hypex better, maybe not. I love Rogue’s true tube amps - I’d encourage you to look at a Cronus Magnum (any version), even if it’s more tubes to manage. That should give you "different" sound for sure! Rogue's amp section in the Atlas / Cronus and ST100 is a crazy value for the money, for a 100% USA-made amp! They sound awesome. 

You should consider a used joule electra amp.  I've had many and they were excellent when I had Merlins.  There were never many joule amps made but they are quite special if you can find a nice used one.

If you are wanting to go with Rogue, you should take a hard look at the Cronus Magnum III. I've heard that multiple times with Maggie 20.7's and it was nearly as good as the $50K Audio Research stack that was next to it.