After installing an OmniFi DMP-1 in my Audi A4, I noticed that that it was overdriving the CD changer input, giving me a slightly noisy sound floor and some noticeable overdrive (distortion) on loud audio tracks. I decided to try a couple of line-level audio attenuators to see if they would help.
I initially purchased the Peripheral Attenuator which is intended for use in car stereo installs. It is a single plastic box with RCA input and output leads. The Peripheral doesn't have any rated db reduction. Instead, it simply has two manually adjustable potentiometers (one for each channel). Because there was no reference for the adjustments, I set the pots at 50%. With this setting, it worked as advertised, reducing the output of the OmniFi and eliminating the overdrive. However, it also noticeably affected the sound quality. It seemed to suck the bass right out of the OmniFi, and slightly collapse the stereo separation.
I then ordered a pair of Harrison Lab 6 db Attenuators from Parts Express. What a difference. Rather than being adjustable, the Harrison attenuators are strictly rated at 6 db reduction (they sell other ratings too), and physically separated per channel. Each is a solid metal cylinder with an RCA male output on one end and a female input on the other. Like the Peripheral attenuator, the Harrisons eliminated the overdrive from the OmniFi, but were totally transparent in sound quality. Bass output and stereo separation are excellent.