i remember back in 97 or so when we were using a motu 2408 and we went out and bought the ad-8000 and stuck it in through the adat pipe. that was a conversion difference that was very noticeable, to say the least. I don't know that apogee and cranesong can be compared to mid-level and Manley given top honors. but 2408 mk1 and yamaha O2R can be given close to bargain basement honors. their newer model stuff may be better, but its not getting picked up by mastering houses so I am skeptical. still good places to start, so that when you do get the top line, you have heard a mediocre unit long enough to know what low end conversion sounds like.
if you have a great converter to A/B, i think the best test to fully realize its powers is to use a source that you are familiar with, such as a demo singer that is used a lot. have him or her do the same passage with the same chain, just change the DAC's and then have them do a few takes with the old one, etc. I thnk this will show off the converter, especially if you are familiar enough with the character of their voice. YOu should hear not only timbral/eq-type differences, but dynamic effects as well.