Hi
What size rotors are you using front and rear?
If you have the BMW (260mm) front and Audi (245mm) rear the you have a 70/30 front to rear bias - this should be able to be fine tuned to achieve the desired balance!
Are you sure the Volvo callipers are working 100%? Have you made sure you have "brigded" to all 4 pistons on the Volvo callipers? If not you'll only have half the volume and reduced braking effort on the fronts. If this is the case you'll have a 53/47 split and causing the rears to lock prematurely.
Swapping to a 0.625 front only you'll have a 74/26 split which may be pushing it a little far forward. Placing a 0.625 front and rear should provide an approximate 66/34 split.