To some degree, efficiency can have an effect, but I wouldn't base my decision of a certain speaker on that. If you really like the speaker, don't pass it up based on the efficiency. If you really need to, you can always add more amplification later. Most mid-range receivers, should be able to drive this speaker. Of course, more amplification is never a bad thing.

As for the 850's, due to the (4) midrange drivers, they offer a little more refinement in the midrange area. It's great for home theatre. Most of the audio track will fall in this region. However, for music, the 760's do great. Put something on with a nice kickdrum and they sound FANTASTIC! If you are using a sub, the bass isn't a big deal though. it's really the mid-range advantage of the 850's that's the deal breaker. You have to decide if it's worth the extra expense. Personally, I think it is, so I went with the 850's! But honestly, both of these speakers do an excellent job with music and movies.