Robson Opulus rated as 8ohm nominal, 5ohm minimum impedance, therefore, no steep impedance curve. 86db sensitivity pretty low. Still, the Evo 200 should be driving these speakers pretty good, just need to turn up volume pot a bit more than usual.
I have PL Dialogue Four, diy modded with boutique caps and Takman resistors. Even in stock form certainly wasn't reticent in midrange to extent what you're hearing, I also own custom built 300B SET and 845 SET amps to compare with. I'd wager you have worn output tubes, and KT family may be best for those speakers, EL 34 most romantic of any tube for that amp. otherwise just not good match. If it was me, I'd try KT in the Evo before giving up, that integrated should match well with impedance of the Robson, just need more volume which the KT family will give. Again, the EL34, worn or not may be your issue. Buying another tube integrated in the price range your talking about isn't going to make some sort of sea change.
I have PL Dialogue Four, diy modded with boutique caps and Takman resistors. Even in stock form certainly wasn't reticent in midrange to extent what you're hearing, I also own custom built 300B SET and 845 SET amps to compare with. I'd wager you have worn output tubes, and KT family may be best for those speakers, EL 34 most romantic of any tube for that amp. otherwise just not good match. If it was me, I'd try KT in the Evo before giving up, that integrated should match well with impedance of the Robson, just need more volume which the KT family will give. Again, the EL34, worn or not may be your issue. Buying another tube integrated in the price range your talking about isn't going to make some sort of sea change.