I'm using the Concentra II with ProAc D15 and have to agree with Keithr on the synergy of the two.
The newer Proac's have a downward firing bass output which makes positioning less of a headache.
Here's what they say on the website.
"Bass loading is achieved through a system which has never been used before in a Response range model. Four large conical feet support each cabinet, allowing the flared equalised ports to couple the bass output efficiently to the floor through the plinths. As a result this driver excels in areas of control, power handling, openness and midrange detail"
The newer Proac's have a downward firing bass output which makes positioning less of a headache.
Here's what they say on the website.
"Bass loading is achieved through a system which has never been used before in a Response range model. Four large conical feet support each cabinet, allowing the flared equalised ports to couple the bass output efficiently to the floor through the plinths. As a result this driver excels in areas of control, power handling, openness and midrange detail"