I have a pair that were made by Tara Labs. Very good for use with Subs if you don't have 2 preamp outputs. It isn't uncommon, you just need to order it. The brand is more a concern about system synergy than anything else.
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?