Turntable-Amplifier-Speaker configuration question.

Or use the phono out on the Technics to the Phono input on the Denon.

Looking at picture of the Technics SL-1500c online there are two pairs of RCA outputs: one labeled PHONO OUT and the other LINE OUT.  To use the built in phono preamp I think you want to plugged into the LINE OUT outputs.  Make sure the LINE OUT switch is set to ON.  That should do it.

PHONO OUT Is likely for connecting to an external phono preamp.

Are you sure that the switch in the rear of the Technics is in the correct position? That cartridge has enough output !

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.

@nickb64 

For us to understand the electrical requirements of your system, we need more information like brand+models:

1. What cartridge are you using?  Cartridges differ in their electrical output 
2. What phono stage (preamp) are you using?  Phono stages can differ in their electrical output amount of amplification from the cartridge output to line level for a preamp.
3. Are you using a preamplifier before your amplifier? If so, what’s the brand and model?
4. What is your amplifier?

Are you using the built in phono option on the Technics? 

@nickb64  -  You stated "Phono to AUX routing".  Can you clarify if that means you are going directly from the turntable to the speakers Aux input; or if that means Phono preamp to speaker Aux input.  If you mean the former than the  issue would be not having a phono preaamp and one that appropriately matches the cartridge you are using, whatever that may be as you didn’t mention it