PrimaLuna Evo 200 underwhelming

If you feel it lacks dynamics swapping to KT88's will help open things up. I much preferred KT88's over el34's and my speakers are also 86db. I'd try those before KT120's or 150's to stay within the comfort zone of the rest of the circuit. You also mentioned that it sounds muffled which can be a characteristic of the preamp tubes, in particular the gain tubes. I'd start by swapping the gain and driver tubes if you don't have any extra 12Au7's on hand, but it's worth experimenting there as well.  The preamp stage has a huge impact on sound quality, particular transparency, so don't overlook the small tubes.       

Also:
Tech at Upscale audio said I needed a full 200 hours for the Dialogue Premium to break in.
I honestly couldn't see it covering that much ground in 50 hours but maybe I was being hasty. Anyway, glad I traded up.

I demoed the Prima Luna Evo 300 against four other tube integrated amps; see https://forum.audiogon.com/discussions/integrated-tube-amp-demo-follow-up. In comparison, I found the Evo 300 to be somewhat veiled, or "muffled", in its sound just as you have. 

BTW, I have now moved on to a Pass Labs XA 30.8, with Conrad Johnson preamp, which is another giant step up in sound quality.

Thanks again for your comments and suggestions. There is a lot to take on board, so please forgive me for not replying to each response, I am not sure how to do that but I appreciated every one of them. There seems a lot more to owning a tube amplifier than I ever could have imagined. Thanks.

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.