Popular opinion suggests a linestage should do little other than provide source switching and volume control. It should add/subtract nothing sonically, and just be a transparent conduit between a source and amplifier. That specification may or may not suit your system. If it does, then a Wyred 4 Sound STP SE with level 2 upgrade is the closest thing I’ve come across to achieving those goals.
It isn’t very sexy, but boy does it sound like nothing.