I threw the sub suggestion out there because you said you wanted to try speakers with a "big and punchy sound" twice.
A sub doesn't have to be cranked up and shake the walls and floors. They actually have volume/level adjustments. They shouldn't be louder than the rest of the system IMO. What they'll do is fill in the rest of the frequency response, and give the Harbeths a bit of a break from trying to reproduce the lower end of their frequency response. A great sub dialed in and set up right can transform a system. Not an easy task at all, but it can be done. It doesn't have to be set any louder than a full size tower speaker would sound.
I'm not saying a sub would be the best bet nor the easiest. Depending on how your speakers are interacting with the room and what you're looking for sonically, it may be a better immediate upgrade than a CDP.
As far as CDPs, it really depends on what you're looking for sonically and budget wise. Have you considered a DAC and using your Oppo as a transport? A DAC can sound just as good, and it'll give you the flexibility of digital inputs. There are a lot of great DACs out there at a lot of different price points.