So I had some time to look at this... very easy to separate. I just added an additional crossmember at the right spot to support the rubber corners of the Concept 30s.
The crossmember I added has stiffened the whole thing up nicely too.
If I had the joinery skills I would have shortenned the length of the whole thing but I was scared I would just end up with a load of scrap, so have gone this way for now.
I could also try bolting the speakers down from underneath as they have the facility for that but I'd need another crossmember or something simlilar.
It was difficult to a/b to do a comparison with the stool tops on, but they certainly don't sound worse and I have a feeling are imaging a little better. Less mass can only help I'm sure.