@bound4h: Firstly, more than probably any other component in an audio system, speakers are a very personal and subjective choice. They are the business end of any sound system. Different ears are going to like different sound signatures.
Secondly, as one very respected high-end audiophile shop salesperson told me: "More power is always better." I would qualify that at bit by suggesting that the power supply and dynamic headroom of a power plant are just as important, if not more so.
Thirdly, Monitor Audio Silver 300 is an 8 Ohm nominal load design with a 90dB sensitivity. That's very sensitive, relatively speaking.
Fourthly, I spent approximately 3 hours of serious seat-time in a scheduled and orchestrated critical listening session with those speakers, the Silver 500s and Focal Aria 926. I was very impressed with the 300 and just a tad more so with the 500. However, the power plant used for that listening session was a monster Delta Classe stack with a heck of a lot more power and much more robust power supply than your Cambridge Audio which, judging from the specs (haven't heard it), is no slouch at all for what it does.
Fifthly, that Rega P6 is a solid (albeit a low mass design) TT. What cart are you using with it? You may find upgrading that might make a significant difference and save you mucho dinero.
Lastly, judging from your core components, IMHO, I'd say you are at that familiar point of diminishing returns, with respect to system upgrades. Upgrading only your speakers will likely leave you wanting and/or needing a better power plant and updating only the power plant will likely leaving you wanting and/or needing more sophisticated speakers. If your present inclination is to upgrade only our speakers, I would suggest you spend some serious seat-time with highly efficient or sensitive ones (e.g. Klipsch; Paradigm; Revel; PSB; etc.). Chances are good, I think, that if you aren't particularly impressed with the Monitor Audio sound signature, the other British designs will leave you even more underwhelmed. Only your ears can lead you in the right direction.
Enjoy the journey!