It could be an artifact of using teh PL tube amp with the KEF ls50. The KEFs are not tube amp friendly. Look for measurements on teh KEFs. Impedance peaks and dips and/or pahse angles at various frequencies with the KEFs might produce teh results you hear. I would only use a high current SS amp with those speakers for teh best results.
I ran a friends LS50s off TAD hibachi amps I had that are designed to emulate a tube amp. The results were much as you describe. Same speakers then sounded great 15 minutes later running off high power high current Bel canto ref1000m monoblocks (Class D SS).
So I suspect teh problem is with the combo of amp and speakers and not inherent to either one alone necessarily.
I ran a friends LS50s off TAD hibachi amps I had that are designed to emulate a tube amp. The results were much as you describe. Same speakers then sounded great 15 minutes later running off high power high current Bel canto ref1000m monoblocks (Class D SS).
So I suspect teh problem is with the combo of amp and speakers and not inherent to either one alone necessarily.