Preamp output voltage...affect on volume???

Everything else being equal (and I doubt that it is) if preamp a has a lower output impedance than b it will play louder at the same setting.

What are the specs on the preamps? (BTW, 50Vrms is EXTREMELY high for a pre amp, are you sure about that?)
Do they really have the same gain?
Do they have the same taper on their volume controls?
Are you using them both balanced?
Why do you have this same basic question posted in several different threads? I think you'll get better answers if we don't have to follow 2 threads on the same topic.

Perhaps it would be helpful if you listed the preamps instead of having us guess what the problem is?

I checked their web sites but can't find any specs for gain on either of the preamps.

That difference in output impedance isn't enough to make any difference. Until you find out what the gain of the Classe is we are wasting our time speculating about the difference.

The maximun gain is within 3dB, not enough to explain the difference.

The output impedance is essentially the same compared to the input impedance of the amp.

The input impedance of each compared to the output impedance of the CDP should make no difference.

That leaves us with the taper of the volume controls. The only logical explanation I can see is that the louder one's steps are bigger at lower volumes than the other one. Have you contacted the manufacturers to see what they have to say?

If you want to ship them both to me I can measure the gain at the different control settings. You could do this yourself with a test CD and a cheap multimeter if you really want to know.

Thanks for the follow up. It helps in the sense that when it comes to gain you now know what you are looking for in a preamp. If you want to stick with the rest of your components you either need one with a lot of attenuation like the Classe or you will need extra attenuators.