I recently completed a set of GR Research AV-1RS surrounds. I like the way they sound and the rear wall mounting eliminated the need for speaker stands and unsightly wires on the floor. I ran the wires through the attic. I had surrounds on stands on either side of my couch but the sound was too "localized". The AV-1RSs blend with my fronts and give a more realistic surround sound. Not sure how they would timbre match with your existing speakers though. Here is a link to a forum discussion on this speaker.
Thanks, They look like nice speakers and the data looks impressive. However I noticed on the GR web site taht they work with an optimum distance from the ceiling of 16". My ceiling is 17' so Im not sure how things would work out. I may try the Audax surrounds and see if they sound okay. At least the voicing would match. Thanks again for the feedback. Anyone out there have the Audax HT diy?
The timbre matching requirement is IMO overstated. I would avoid mixing drivers that are very different in sonic characteristics such as metal and paper cones. However, I'd say you could safely match paper, polycone and similar technologies.
I have seen a review in an online audio magazine (like Stereophile, not sure which one) where they compared different types of surrounds - monopole and dipole in different configuations, with different numbers of speakers. General consensus was that dipoles were preferred most of the time.
M&K's tripole concept seems promising. They use a direct radiating monopole which has more bass response, then smaller dipoles. It gives you a controlled mix of the benefits of dipole and monopole.
I have had a couple of different ideas for surround speakers. One is put a pair of bookshelf kits back to back to get dipole - simple from a design point of view.
You could make some simple dipoles with a fullrange driver like the TB W3871 and cross it to a midbass driver like the Vifa P17. The fullrange drivers de-localise in the midrange and make things simple so you don't have too many drivers. The midbass fills in the lacking bass and lower midrange, giving them better dynamics, but you don't need a big box. You could extend the concept further by adding a tweeter and bi-amping. You can then adjust the gain to each and choose the balance between monopole and dipole. You could add a gainclone to drive the fullrangers. You could also use active filters to shape the power response, which is a critical factor on surround speakers.
If you are a little clever, you can use a smaller midbass (5") and design for less extension but more dynamics and output in the bass if you cross at 80 Hz. You design for an ugly looking bass hump just below 80 Hz where it is tuned, but you filter out this hump. The result is that there is very little cone excursion around tuning, even less than usual and below this point the crossover reduces excursion. What you get is about 12db more output so a little 5" driver can rival a bigger driver which would need a smaller box!
How to do this can be found on Elliott Sound Products - articles - loudspeaker section - one of the new articles.