The revisions centre around lowering tuning frequencies to take advantage of the larger cabinets (EP125/175) and the improved parameters of the new woofers. To give you an idea of the changes, I have attached the natural frequency response (no EQ) of the EP125v3 (in blue) and the EP125v4 (in green). There is an improvement of 5dB between 25Hz and 40Hz with the new alignment, meaning less EQ is necessary, resulting in much greater low frequency headroom.
Excellent Andrew. That's what I was looking for. Axiom's subs have been a weak spot in a lineup of exceptional value speakers.
If you have managed a similar change in response with a smaller footprint for the EP350 you will have a real winner on your hands.