Thanks for the responses, I'll try to answer all the questions all below:
I am connected from the Phono output of the Technics SL-1500C (that has preamp built in) directly to the AUX input of the KEF LSX II speakers.
The cartridge on the turntable is an Ortofon 2M Red (voltage output 5.5).
The amplifier I tried to use to boost the output was a Denon PMA-600NE Integrated Amplifier.
Thanks again.