B&W 803 D3 speakers - which amp?
An "impedance" vs "-phase angle" graph has never been done on the 803’s, but:
Here are the impedance phase graphs of all of the 800 range, no reason to believe the 803 won’t be the same, which are all very hard to drive in the bass (70hz - 900hz) given their measured impedance and also when it’s combined with -phase angle (EPDR) which can look even lower to the amp that hust the impedance alone.
These speakers need amps that have good current delivery, an owner should seek out amps the "can come close" to doubling the 8ohm wattage spec for each halving of the impedance, if they want to get the very best from the bass as well.
EG:
8ohms= 100w
4ohms= 200w
2ohms= 400w
802 https://www.stereophile.com/images/616BW802fig1.jpg
800 https://www.stereophile.com/images/511B800fig1.jpg
804 http://www.prespeaker.com/images/201702/B&W-804-imp-thumb-650xauto-15553.png
801 https://www.stereophile.com/images/archivesart/801fig1.jpg