I'm looking at doing some playing around to measure frequency response of some equipment. In particular analogue vs digital EQ, and the effect of changing valves (sorry, tubes) in a headphone amp. Can anyone recommend a good USB ADC for measuring the RCA line out (doesn't have to be super accurate, just repeatable, budget say $100-$200) and capture/analysis software? REW sounds like it may be pretty handy. The configuration I'm envisaging is laptop - DAC - device under test - ADC - laptop. May need to have the ADC in parallel with headphones (not sure if the headphone impedance will change the overall frequency response or not, this will be one of the tests).