I didn’t mean to dis your Onkyo receiver. I know that Onkyo is very nice for a receiver and your model is VERY hefty at 54 lbs. (hardly any receiver has that much weight). This is very good for its power supply. I was just trying to read into what you like as far as sound goes. The B&K is going to be very warm/rich sounding (I used to own a B&K 200.5 amp). It was too warm for my taste, but others love it. The Onkyo, on the other hand, is going to be a very clean/clear sounding unit in comparison. It is two different sonic signatures. If you like the Onkyo, by all means try it out and see how it goes. The power supply is extremely large for a receiver. It may have no problem driving the 4 ohm speakers.
That being said, if a speaker is rated at 4 ohms (even some 8 ohm speakers), the actual impedance can drop down to 2-3 ohms in the bass/midbass area. Some amps just do not have a large enough power supply to provide enough current for these impedances. The result is weak bass/midbass.
If you feel that the Onkyo is not driving the speakers well enough or if you want the warmer sound of the B&K, then the B&K amps I suggested would be a good solution. They have a very large power supply for just the 5 or 7 channels (I think they come in at about 63 lbs or something). If you decide to sell the 507, you could fund the purchase of a separate amp.