The level differences are due to the VP150 at 6ohms and the M60s at 8 ohms. Once you get the M80s it is reversed, the VP150 needs to be boosted to keep up with the M80s, but not as much.
I A/B'd M60s and M80s and the differences were mostly noticeable at lower volume listening, the M80s are clean, crisp and clear at low volumes(all volumes actually) while the M60s I found to be more subdued. Once volume levels reached higher ~80 - 85db and up, the differences were negligible as the M60s really opened up and sounded more clear, YMMV. If you look back into late 2007 posts, you will find some where mojo and I did the testing.
