For low mu valves, it has not been my experience to find close matches easily. For GE 5-Star 5687, they are soo close as to deserve 'perfect', but that's a whole 'nother pipe( errr...tube ).

The mu of 2-2.5 achievable with resistive plate loads is appealing. The little bit more gain than is available from a buffer circuit is just what the doctor ordered. I do hate the idea of throwing away gain like I would if I build around a crappy 12AU7. With the mu of 6 from the 12B4, and 1 from a buffer the 6C19 looks like Baby-Bear's porridge...just right!
cheers,
Douglas