I don't have any other recommendations, unfortunately. The Q+Q are my only experience with tubes. The Dark Voice is an OTL, so probably won't work so well with planar headphones. The PM-3 have a flat impedance curve, so impedance mismatch shouldn't be a problem (I don't think; don't quote me on that), but OTL deliver more power into higher impedances. Are you talking about the Cavalli Tube Hybrid (CTH)? The power stage of that is solid state, if I'll not mistaken, so it may work. Again, I have no actual experience, just going with what I've gleaned from the internet. Oh yeah, and if you like bass, the bass on the Bottlehead setup with the PM-3 is just... Oh, mama!