The closest I have seen is Dennis (Tharkun)'s comments on a listening test he did between M2 and W2s up at Axiom. I think he ended up preferring the M2s but the difference was very small.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8