After some tweaking I got the response as depicted below. This was measured using REW in a room, so full of reflections. I had to apply IR windows which affect the low freq data somewhat. Quite happy with the phase response now (note: with IIR non-lineair phase filters, DCX2496). Settings:
MID: 100 Hz BES12 HP, EQ -4dB Q=2 352Hz and 676 Hz BES24 LP.
HF: 676 Hz BES24 HP, EQ -4dB Q=2 3.5kHz and 6.5 kHz LR12 LP (BMS passive C8-8 filter)
VHF: 6.5kHz LR12 HP, inverted (BMS passive C8-8 filter).
So, you would expect 90+360+180 degrees phase shift due to the filters. At 100 Hz there is an additional 90 degrees phase shift due to the BR ports. So, this is about the best phase response you can get out of the DIY with IIR, I think. (I don't want to take the risk of damaging the BMS by going to a 12dB/oct x-over instead of 24dB/oct...)
For further EQ tweaking I will need to get the set in open air (or a very large room).
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