It is possible you enjoy a brighter sound signature than the UB5’s produce. It’s my understanding that some early Elac models rolled off the highs a bit. I doubt what you’re noticing is the fault of the Yamaha. Yamaha has a really good reputation with their amplifiers.
A primer on room treatments:
Admittedly, he’s shilling for his company, but there are good tips in there.
Here’s some cheap acoustic panels if you want to play around with them:
If none of that helps and you want to go the amp route, @ZeosPanera reviewed some Emotiva class D monoblocks that he said have a brighter-than-average sound:
Fortunately for you, the SU8 has volume control. Also, it has some EQ presets that you could try before any of this, come to think of it.
If you decide to move on from the Yamaha and/or Elacs and are willing to part with them for cheap, lemme know