They work superbly, I have one and it's saved me countless hours while greatly improving performance.
http://www.kabusa.com/preconlp.htm
Hagtech makes a similar device for breakin:
Inverse RIAA Filter
ok to break in phono preamp w/ non lp source?
I just got a brand new Vincent Pho-8 phono preamp. I would like to break it in using my Linn Classik since I can leave it on without changing records and my turntable is in the shop until next week (unexpected delay, sigh). Would it be ok to have my Linn Classik set on the tuner and use either the tape out or preamp out (set to low volume) running right to my Vincent phono preamp? Some people have spoke of reverse RIAA filters but hopefully that is not needed.