Think I've hit the sweet spot eventually.
My overall aim was to:
1. Maintain a full open baffle design
2. Eradicate or minimise crossovers and pass filters to avoid compromising the sound quality of the all-important midrange.
3. Feed everything from a single valve amp
4. Get unnoticeable overlap of bass and mid drivers to avoid the muddiness I had observed earlier at the overlapped frequencies
5. Match the outputs (volumes) of the two drivers.
So looking firstly at the 8" Cube Audio FC8 it turned out to be slightly less sensitive than the vintage 15" Altec 418. Playing on its own the FC8 had audible output down to 200Hz and needed no filters - nothing.
The Altecs were slightly too sensitive (loud) so required a little attenuation and a second order LC low pass filter at 160 Hz. This was achieved using a 10mH iron core inductor (0.6 ohm), together with 100uF cap. This filter resulted in the Altec just gently kissing the FC8 - no gap, no noticeable overlap and evenly matched volumes.
Sum total of all electrics per speaker is one quality inductor and one quality capacitor
For the first time in a year since I started this little project I am happy
