So, Plan B failed too, so I bit the bullet and bought a Celestion 5" speaker, in 8 Ohms, the Truvox TF0510.
Shame the mounting centres are larger (why would they do that? -moral- never assume!) out with the file. It does sound nice now though.
I did try bass through it, and it sounded terrible. There were a couple of screws missing and the box isn't exactly airtight, so it chuffs like a goodn.
I filled the screw holes and it made a huge difference, now perfectly usable for bass in the house, and a bit of sealant/gasket for the rest of the box should eliminate all the rest of the unwanted noises. A proper result in the end! I'll try it out at the monthly jazz jam next sunday and see if it keeps up!
By the way, the amp is a Trace Elliot Acoustic Cube, a 7" metal cube. It's very nice with acoustic guitar, and violin too apparently.