impedence value.? amp to preamp.

The concern of resistor based attenuator is that the output impedance changes
a lot depending on the setting. That means when matched with a low input
impedance amp, your system may sound very different as you turns the volume
up and down.

The problem with a lot of passives is that the input impedance of the amp interacts with the cable capacitance and the source impedance of the volume control.

The result is that there is reduced bass and impact at low volumes. Passives really work best when they are built-in to the amplifier.

05-05-09: Sidssp
The concern of resistor based attenuator is that the output impedance changes
a lot depending on the setting. That means when matched with a low input
impedance amp, your system may sound very different as you turns the volume
up and down.

I'm afraid that I do not quite agree with you. For example, here is one implementation of a resistor attenuator:
http://www.tweakaudio.com/Attenuator%20info.html
The only places where the impedance will be high & is likely to affect the sonics is at the -0.5dB, -3dB, -5dB, -7dB settings. Of these settings the -7dB setting is probably the best as the effective impedance the power amp would see in the 500 Ohms region. This is still high for a 10K input impedance power amp but it just *might* work - on the cusp.
(BTW, a digression: my calculations show the 1M resistor provides -0.008dB attenuation, the 4.99K provides -1.5dB attenuation, the 1.69K provides -4dB attenuation, etc. Am I missing something here? Is he adding some other resistor in series w/ the 1K such as the source output impedance (which is not shown)?)
More often than not, the user is going to be in the -31dB range where the effective resistance of this attenuator will be defined by the resistor to ground - which is a very small value. Thus, for even a 10K input impedance power amp this should not be an issue.
(this is all electrically speaking, which is the focus here. No idea what the in-line attenuator does to the overall sonics - my intent is *not* to discuss that here).