The XLR versions of the Rothwells and Shure had the same "issues" (with my Nuforce separates) as the enjoythemusic.com review reported about the Rothwells. The Endler attenuators were much the same.
My experience with in-line attenuators also includes the Harrison Labs (apparently only available as RCAs at parts express #266-242) which have merit.
But goldenjacks.com attenuators (no affiliation) were much better. In my Goldmund rig, goldenjacks reduced the volume without objectionable side effects. They currently offer RCA in 6db, 10db, 14db, and 20db versions, with an XLR version expected to be out very soon.
The Rothwell and goldenjacks.com websites have good write-ups on how and why attenuation can enhance listening to CDs.