i believe that the issue with your system is that your power amplifier is inadequate for your KEF LS50 speakers. Looking at the impedance & phase plots of your LS50 on Stereophile I see that your speaker has some really crazy phase ups & downs making it a challenging load for your amplifier (or any amplifier for that matter). Couple this with the rather low sensitivity of 84.5dB/2.83V/1m & it means that you basically need a high current amplifier to drive this speaker if you want it rock. Looking at your Sony integrated amp I don't see it as being a high current amp - it's not high/tall enough & it's too lightweight. This Sony is more of a life-style product than a serious amp for hard-to-drive speakers like the LS50. This is also confirmed when you wrote 8W/ch into 8 or 4 Ohms. So, while it might have 80W to offer, it has very little current to offer. If you cannot get high current into a low impedance you will not get a high enough voltage. And its voltage you need to generate pistonic motion of the speaker drivers to generate the SPLs you are looking for.
Either get rid of the Sony amplifier OR get rid of the LS50. These items are mismatched.
Either get rid of the Sony amplifier OR get rid of the LS50. These items are mismatched.