Passive preamps: What are the relevant numbers?

I'm not sure what your amps input impedance is, but if it higher than 40Kohms you shuld be fine

Do you consider this a general rule of thumb where passives are concerned, that they feed into loads of >40K ohms? Why is this?

I got that as "general rule of thumb" from the manufacturer of my passive - but I don't know the technical reasons. I do know that the higher the input impedance of the amp (the 100kohm typical of tube amps seems to be ideal) the better, it is - an easier "load", and the lower the output impedance of the source the better (my DAC is 200 Ohm output impedance). The other factor is adequate voltage from the source (CDs are typicall 2 volt and up - more than adequate for most applications) and a low input sensitivity for the amps (I have a 1.5v and 835 mv sensitivity on my two amps, the 1.5v amps are more than senstive enough). It is probably ideal to have the output voltage twice the amount of the amps input sensitivity. The you want low capacitance in the IC going from you pre to the amp - I read somewhere that less than 100pfs for the run is ideal.

Dave: the 0.5V you are looking at is the digital output. Line is 2V for most Rotel CDPs - standard. You are fine with a passive pre into DNA - keep the interconnecs not too long and of low impedance.

Well, my wife and I just finished the Bolero Test as described by Arthur Salvatore at high-endaudio.com. Basically, we played Ravel's Bolero directly from the Rotel CD player to the Amp and bypassed the integrated amp completely. Our findings playing Bolero directly from CD player to amp:

1. The volume was much louder than our normal listening level.
2. Better separation of instruments.
3. No seeming lost of bass.
4. Horns seemed to be a little sharper. Brighter?
5. Sound seemed more natural.

We went back and forth a few times. The first time through, my wife didn't know what I was doing and made similar observations to the ones above. I preferred the sound without the integrated amp, although the horns were a bit sharp.

I'd be interested in doing this as a blind test, but we both thought the differences were very obvious. Will placing a passive amp in between the CD player and the amp likely result in similar results?

By the way, as recommended by Arthur Salvatore, we only played Bolero because it starts out soft. I was afraid to do it with another piece of music because the volume direct from CD player to amp was so loud.

There are several different types of passives. There's no real way to know how one would sound in a given system. You'll have to experiment.

Good luck