Which volume do i use?

Taking a safe approach: Connect the CD straight into the P.A. then crank up the attenuator to the loudest level you'd ever want to operate at, or until clipping, whichever occurs first. Note that CD attenuator setting. Then when you connect through the preamp, set the CD attenuator to the previously noted value & control levels from the preamp.

I ran my CD-55 at "99", which I believe is what is recommended by Resolution if you are using a pre-amp. At enjoyable listening levels it allowed me to keep my pre at ~50% output. I didn't do a comparison of the CDP attenuator's response vs. the pre-amps, so I don't know if there is a sonic benefit.

If I am not mistaken, you should try and set your pre-amp volume controls so that you minimize controlling at the extreme ends. Linearity of the volume control response? Maybe someone else can answer that.

Massvm, What do you mean 99?

At "0" attenuation, or the highest output, the volume display on the CD-55 indicates "99". Sorry if I wasn't clear enough. Maybe the CD-50 is different?