Have been using a PMD661 for a for some time now and have used both the 1/8 Line 2 Inputs as well as XLR without a problem. Usually the Line 2 1/8" input was used when I had to use an RCA Tape Out from a board. I always try and use the XLR In's on the 661 if I can, but now that I purchased a Sound Devices MixPre-D, I have only been running the AES out of the MixPre-D to the S/PDIF of the PMD 661 with amazing results!