Okay, so I just played the Prelude to Das Rheingold(Solti, VPO, etc.)and see what you mean. Different recordings of it have different bass levels there, but a sub does help. Nevertheless when I played the M22s without my sub(as usual)I got a good sense of the weight of the music and was reasonably well satisfied.
Incidentally, since you're interested in vocal music, let me suggest an absolutely gorgeous Rachmaninoff Vocalise et al with Anna Moffo singing and Stokowski conducting on RCA 7831.
As Ken mentioned, check those willing to show off their Axioms. In particular, note that Peter(pmbuko)is in Oakland with M22s and SVS PB 1.
