It's kinda ironic that the subject of big diameter speakers for use with guitars turned up in the forum at this time.
Just last night, I googled Ampeg B-18X, which was my personal dream amp for combo organ back in the 60's. I could never had afforded one then, and in fact, had never even actually seen or heard one.( There were many bass players in my town at that time useing the classic Ampeg B-15 and B- 12s)
I was surprised to lean that the designers at Ampeg actually had guitar players in mind when they created the B-18X! The X featured a small horn to reproduce highs.
here's a article on the B-18X. (maybe if my ship ever comes in, I may still get one, they seem to go for about a grand, but with a fresh set of tubes, I betcha' it would kick ass hooked up to my synths.)
http://www.ggjaguar.com/b-18x.htm
later,
Howlin'