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

No. It won't sound louder. By using the attenuator you will have effectively reduced the output to the amp to the same level, you'll just have a lot of power in reserve that you will never use unless you have a grossly insensitive amp or one specifically designed for that pre-amp output (balanced design perhaps?).

No, it wouldn't be louder. In fact, I am willing to bet you wouldn't even use any of the gain in either one! Many preamps don't need to be set past unity gain for the loudest music level used. Exceptions are preamps with post attenuators or those mated to a highly insensitive amp, like Newbee says.

Arthur

Ok, so if this is the case, why does preamp a (50 vrms) play significantly louder than preamp b (20 vrms)? Is all due to the gain??? I am really confused because the only thing I have changed in my system is the preamp I am auditioning and the sound is already above my listening levels at volume setting "1" on the preamp (preamp a). Please explain so I can better understand what is causing the signficant difference in volume I am hearing.

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?

herman, I currently have the Classe CP-700 pre. The other pre I am auditioning is the new BAT VK-52se.