Rogue 100 dark

With a SLP-98 and proper speaker placement in a good room, you should have everything you need. The benefits of tubes happens after you have everything else sorted out. I'd look at setup rather than gear. 

Roll in Some mullard Input tubes and golden dragon Output tubes

I found the Rogue had a tubey sound just not that older classic tube sound. Rolling in better 12au7 and 12ax7 improved it a lot. Well at least it did on the stock ST100 and not Dark. Like the above poster mentioned look at your setup. The ST100 is a very nice amp with plenty of power.

Not familiar with the Dark version but enjoyed the ST100 for few years mostly in Triode mode. Toggle switch in the back. Give it a shot. 

this amp has great nos tubes including the mullards you speak of. I think my setup is fine. At least it is using the cary ss amp. Power is not the issue here. It just lacks the subtleties in the mids. i am using it with the switch in the down position. I have not tried it in the up position. I'll try.