Just read through this thread and I think there are some good options. 1st of all I would say stick with the M2 or M22 over the M3 to better match your M60's. The full bookshelf would be preferred, but since you don't have room under, I'd see if you can make one work above - maybe Mark would like to do some woodwork out west?
If above the TV does not work then I think you could try the I/O M2 with some mounting board below the TV as you suggested. Note that I have not heard the VP100, but my experience was that the M2 performed better than the VP150 in my setup. Plus the general consensus 'on the line' (as my 5 year old was saying this weekend) is that there is a gain to be had going to a vertical center all other things being equal.
If nothing else it will be a fun experiment!