Amps AMps AMPs AMPS! Need advice...

I suspect a diamond speaker works along the same lines an an open backed guitar amp cabinet. There is some bass attenuation, varying depending on what's behind the speaker. That's what I use (open back, 12 inch speaker) for both theremin and guitar. My bass amp has a ported box.

Notice on Peter's video with a naked speaker, he has a microphone stuck right in it's face, so the recorded sounds should be a bit more full range.

I'd try your Fostex speakers and see what happens. All you have to loose is a bit of time and a small amount of plywood.

A Peavey BW, while a great speaker, would probably be overkill here.

I use a naked speaker because the lack of a baffle actually weakens the sound, giving it a more human and fragile timbre. 

I was wondering if anyone around here has any experience with loud speaker design; I got a older pair of fostex L475 15" 150watt 8ohm woofers for free (similar to a fostex fw405n), they seem rather nice, and was thinking about building enclosures for them. Doing research today is making my head swim.

I am very familiar with the speakers you have. They are DEFINITELY not suitable for an instrument amplifier. I have some ideas for your application and would be very happy to help you. I tried reaching you through what I thought was your Facebook page with no luck. Please send me an email directly at cavu at flying-colors dot org

Michael.

Regular amps/ speakers are low, on the floor whilst the diamond speaker raises  the audio source  and is traditionally placed directly behind the player's head.    This gives her a clear split-second 'preview' of the signal.  Also, having the speaker right by your ear (relatively)  allows one to  fish around for the right note while keeping the volume really low and the audience wont hear it.