I am currently running my Quattro through a similar set up, where it is used solely as a DAC for a dedicated amp. I've also been switching back and forth the Benchmark and Matrix with a set of monitoring IEMs and once again have found it difficult to distinguish between the DAC functions of the two when the audio signal has been output to my amp (it's a tube as well).
If you're looking at brand new units then go for the Quattro. Although I've seen used first gen DAC-1 units on auction for about $650-700, I find having a USB input on the Quattro makes it more versatile. All in all both DACs are evenly matched, even if the Benchmark has more of a show-off factor attached to it
Edited by xjaynine - 8/25/11 at 6:25am