You may want something better since you're using Levinson, but I've good results from my Cobalt Cable Y's. Very cost effective. I bought a pair of these to use with my Norost SPM interconnects while I saved up for another interconnect pair for the second amp. Now I just use both amp out ports on my preamp which I think is better. I do think I get more background detail now, but I wouldn't say these Y's dimished the sound all that much. I'm sure the Purist would be even better.
Balanced Y Adapters--Where to Find
The manual to the Levinson 380S pre specifically discusses the use of Balanced Y Adapters for passive biamplification. I have no clue who makes these or where to buy them. Can someone offer direction?