Just a bit of an update.
I have the 15" bass unit disconnected for now until I work out a low pass filter.
The tweeter has its existing capacitor to protect it and the SEAS Exotic W8 is running full range (both top and bottom) - no capacitors, no inductors, just silver wire to connect it. This is by far the best it has sounded.
Essentially this is now a two way open baffle and the best it has ever been. I could live with it for a few months if I have to. The vintage Tweeter and Exotic W8 work beautifully together so looks like I fluked that.
The next job is to integrate the open baffle bass driver into the system, just like one would a subwoofer. As there is no high pass filter imposing a defined slope on the midrange driver (eg Butterworth 12dB), integration is that bit more difficult and continues to insist I buy a calibrated microphone and do some measurements.
Maximum flexibility seems to be the order of the day so a DSP amp is attractive, but it needs a high level input as the system volume will be controlled by the 211 triode integrated. This is very common for subwoofers, but not DSP
Here is a spec for what I am looking for.
1. High level inputs from speaker terminals of main amp (therefore no impact whatsoever on the existing two way drivers)
2. DSP for maximum flexibility.
3. Amplification should be best SQ but probably can cope with 50 W/channel minimum. (Class A/B or D Class?).
4. No thump at turn on.
Will try and post a YouTube video later though I dont yet have a good microphone.