FWIW, I use a Weber Z-Matcher with my two marshall cabs:
Jcm900 1960A (w/ V30 and WGS Retro 30 speakers at 4/16 ohms)
Jcm800 1982B (w/ g12-80 speakers at 8 ohms)
(I usually run the head at 8ohms for two 16ohm loads, so the 1982 goes through the z-matcher to make its impedance 16 ohms)
It does its job and i don't hear any difference at all with the tone when going through it.
The line out feature is great for slaving into other power amps for that big marshall wall on bigger gigs. I highly recommend it.