I'd love to hear the opinion from someone who has heard the 160 and 180 in the same environment.
I have heard both in my home at the same time. I found the VP180 was not as good for off axis listening in my room but it offered slightly better lower mid range performance than the VP160, ie. the gunshots in U-571 during the boarding scene had more impact with the 180 vs 160. If I recall there were some other minor sonic differences much like M80 vs M60.
Prior to my 180, I had the 160 as well and I found much like yourself, the 160 to have a little better off-axis performance than the VP180. I believe the design difference, although subtle, is to provide slightly better vertical(up/down)sound performance in the 180. Although I never listened to them at the same time, based on my best memory and to my ears anyway, their differences are really quite minor. The only reason I made the change was that because at the time, there was a sale on custom finish upgrades(my VP180 is piano gloss black)all to match my L/R M80s along with receiving real good trade-in value for my VP160, otherwise, I really doubt I would have made the change. So, in other words, strictly to satisfy my vanity.
Now that I am going through a major entertainment/rec room renovation, for the sake of handling ability, weight and space, in some respects, frankly, I wish I had the 160 back.
