PrimaLuna Evo 200 underwhelming

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.

There is an interaction between your speakers and the tube amp because the tube amp should be strong in the midrange unless a mismatch occurs, which is what you are experiencing.

Thanks for digging up the info on my speakers and also for the mismatch info. I have tried to swap around the input tubes as suggested and I can’t budge them. I am frightened to use too much force incase I damage something I thought they should easily pull out but I seriously can’t move them.

Try gently rocking them back and forth in all directions while pulling upwards.

PL has a warm, pleasant sound. To my ears, it was not too detailed. It is a good amp in my opinion to mate with modern loudspeakers that are detailed, analytical and bright as it balanced those things out nicely. 
But I too found that over time PL was not exactly what I was looking for.  I would, however, recommend trying different power tube variants if you can afford it.