PrimaLuna Evo 200 underwhelming


Hi all, I wonder if someone can help or advise. I bought an ex. Display Primaluna Evo 200 a couple of weeks ago, mainly based on all the rave reviews and excellent comments on various forums. I must say, that vocals are terrific but overall, the sound seems muffled. There seems to be a hole where the mid range detail should be. I have tried the 4 and 8 ohm settings and there doesn’t seem a great deal of difference. I am using it with a Linn Genki CD player and Robson Acoustics Opulus speakers, also Vintage Tannoy Mercury speakers. I haven’t messed with the tubes because I have no idea how to do that. I so want this amplifier to sound beautiful but there is something missing. Can anyone help please?
golfnut2
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.
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.