Thanks for answering me John, I appreciate it!
The system you linked to is quite close to what I have in mind except that in my case the woofers would be positioned horizontally and very close to the floor..
I need to keep the system as invisible as possible. The bench would be white on all sides with the woofers covered by acoustically transparent cloth. The electronics would be placed on shelves behind doors between the bass drivers. The whole thing would look like two horns on top of an elongated box. The whole front would be a one big baffle. This system is for our living room.
This is a rough sketch of the bench that I have in mind (front view):
_o_________o__ <– horns
O O O O <– Dipole15
The bench itself has a wide front and top. It can have sides. It doesn’t need to have a bottom or a back and it can have internal bracing or whatever is needed. As long as it looks as a simple wide bench. A bit like this but with doors for the electronics: http://www.ikea.com/gb/en/images/products/byas-tv-bench-white__0144833_PE304277_S4.JPG
I don’t think that I am ready for a 2.5 system. One of the main aspects of this project is the simplicity. As I wrote, my amplifier has built in 2 way crossovers. Going 2.5 would increase the complexity very much. Or could the 2.5 part be implemented with passive components so that the amplifier still would see the system as a 2 way system? An extra power amplifier would not be a problem but I don’t like the idea of dealing with complicated external electronic crossovers.