I just wanted to give you a better understanding about volume controls that you may see in various DAC devices. The majority of them operate in 32 bit depth, which provides a pretty good degree of attenuation before any dynamic truncation would occur when dealing with standard red book. Its when you get audio sources that are 24 bit where you may experience some dynamic truncation as you only have 8 bits remaining before you could potentially impact the digital source when attenuating the signal digital. Of the devices mentioned, I should point out the NAD C510 does employ the Zetex (now CSR) DAC chip which is its own particular design. Its digital volume control is 35 bit, giving it fairly decent headroom even when dealing with a 24 bit signal.
Some devices use analog stages to avoid this issue and some don't bother at all. I've also seen a blended approach of analog and digital volume controls blended to try and eliminate the digital issues at lower volumes. Investigate each product as I would have some issues with a 32bit digital only control.