Now, it occurs to me as well that if the VP-160 draws less power than both the VP-150 and VP-180 (4 ohm / 6 ohm vs. 8 ohm) and since the sensitivity is also good, that it will be less taxing on the amplifier, leaving more headroom for the rest of the system and for clean frequency response. Am I right on this?
I would say that the 160 will draw more power than the 150, but less than the 180. This is based on the number and type of drivers. The 150 has a lower impedance rating, but has smaller drivers, the impedance rating is determined by how the drivers are wired, the more there are in parallel the lower the impedance will be.. However, the more/larger drivers there are the more power the speaker will want, regardless of the wiring configuration.
Less taxing on your system..
The 160 will draw less power than your M80's, so relative to your 80's it will be less taxing on your amp. The concern that you should have is driving channels of your amp into the clipping region/exceeding the capabilities of the power supply of your amp. You have a 5 channel amp, each channel operates independently of the others. However, they all draw power from the same power supply. You are more likely to drive the channels that are powering your M80's into clipping than the channel that is powering the VP-160. Are you driving any of your channels into clipping? if you aren't then don't sweat it... If you are, then you have 2 options...
A) turn the volume down
B) buy more power (I bought more power)
But, now what about only 1 tweeter. Doesn't this mean it might not sound as good at loud volumes given it lacks the 2 tweeters of some of the other Axiom speakers?
Axioms is one of the few companies that i am aware of that uses multiple tweeters.. It does not take much energy to recreate high frequencies. I have M60's and a VP-180, they all sound the same.
Hope this helps.