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?

Your volume level should be adjusted to match the volume level of the other source components you are using so that when you switch between sources they are all at approximately the same level. IF you do not have other high level sources - forgo the preamp. As a general rule of thumb, you should set the volume on the CD50 towards the high side or all the way up. The CD50 has a LED bar showing levels, the CD55 has a numeric display.

Have you tried running the CD50 without a preamp? It sounds the best this way. The Resolutions have an excellent volume control - it works in the analog domain and does not degrade the sound (unlike the volume controls on Wadia players which sound better using a preamp). With the Resolution, the best preamp is no preamp.