I have an MSB Discrete DAC with a Premier Power Base. it has a built-in volume control, so a preamp is unnecessary. However, at the urging of my local stereo dealer, he lent me an ARC LS28SE to try. I felt that it really added so little benefit that I returned it. However, when I moved up to an ARC REF 6, there was a marked improvement across the board. So does one "need" a preamp? I didn't, but putting the REF 6 in the system really bettered the volume control in the DAC. I think that it is a testament to the quality of MSB's volume control that one needs a 16K preamp to better it, but better it it does!
