I'll start off, then everyone else can jump in and correct me
1. My understanding is that the main difference between M80s and M60s is that the 80s can play louder (ie REALLY loud) and the bass goes a little bit deeper on the 80s. The 80s would probably be OK in a 14x24 room but my first thought for 14x24 would be the M60s. I have M60s and 13x23 room.
2. Don't know much about 8.1, but the main issue for M80s is that you need a receiver which will drive a 4 ohm load well. I think Denon and HK are the normal recommendations... with most other brands it seems like some models can handle the 4-ohm load and others can't.
3. The main difference with M60s would be that a much wider range of receivers would be able to drive them, since they are 8 ohm rather than the 4 ohm M80s. M60s are also a tiny bit less sensitive so to get the same (REALLY LOUD) volume out of the M60s you would need more power.
4. I don't know enough to answer this, but my first thought would be that having all those cabling positions gives you flexibility in speaker choice but that you would never want to use all the possible speaker positions at the same time. In the stereo world having multiple speakers driven off the same signal (eg. floor and ceiling mains) gives you more volume but blurs the imaging so usually nobody even tries. Might be different for HT -- anyone ?
5. My recommendation would be "you don't need R&L ceiling speakers"
6. Again, out of my depth. From reading other posts it seems that they can sometimes add "that last bit of detail" but they would be one of the last things to spend money on. If your area has exceptionally bad power quality it might be a different story.
7. I have never heard an Axiom sub but I have a PSB which is probably in the same league. I think the Axiom subs get a bit of a bad rep because they are just "really good" while the rest of the Axiom speakers are pretty much "absolutely the best thing you can get for the money". If you want the absolute best subwoofer performance for the dollar the HSU and SVS models seem to be ahead -- on the other hand lots of people order the Axioms, get a good-performing sub that matches the rest of their equipment and doesn't look like a big black wife-scaring lump lurking in the corner and are very happy.
Not sure where Velodyne fits in -- the sense I get is that the HSU and SVS seem to be what people looking for the last bit of performance tend to get.
8. Not even gonna try. Will watch the thread and try to learn something about 8.1
9. The whole 8 ohm / 4 ohm thing is always hard to explain from anything but a dry meaningless technical perspective... but let me try. Impedence is sort of "like resistance but for AC rather than DC". If you connect a battery (or an amplifier) to an 8-ohm load then a 4-ohm load you will get twice as much current going through the 4-ohm load, and will get twice as much power and play louder AS LONG AS THE AMPLIFIER CAN PUT OUT TWICE AS MUCH CURRENT as it was designed for (assuming it was designed for 8-ohm loads which is pretty common).
If the amplifier isn't up to driving the lower impedence load then a variety of bad things will happen if you turn up the volume -- overheating, clipping/distortion, or (more likely these days) a protection circuit in the amplifier will detect the extra current and shut down the amp. No music.
Some receiver mfgs believe that the ability to drive a higher current load is a generally good thing (slightly better sound) even if you are not running low impedence (4 ohm, 2 ohm) speakers. This high current design is what lets the receivers handle 4-ohm loads with few or no problems -- and that is why HK and Denon tend to be recommended for 4-ohm M80s.
Your next question is "so why are the M80s 4 ohm anyways ?". I don't know the answer to that one although there are a number of possible answers.
