surely it would depend on the dac. even with a modest x2 interpolating (as opposed to zero stuffing) oversampling you end up with a max frequency of 44.1khz (as opposed to 22.05khz) with only every other sample being guesstimated. at this point the signal (being analogue) can be sampled at whatever frequency you want by the amp, dither added and all the other tricks only usually applicable at the recording stage to help out.

why would an amp upsample anyway surely they'd just sample the incomming analogue signal at whatever rate it uses natively.

cheers

julian