...onkyo TX-SR805...Def Tech Bp7004...C/L/R 3000...BPVX/P.
Even when you use just the Onkyo805 and no external amps, you are in effect bi-amping all those speakers! Great system.
The Onkyo is amping the DT's tweeters & mid-ranges, while the DT's internal amps are powering the woofers----> bi-amping.
The only other thing you can do is buy TWO Y-Spliters (1-male/2-female type). Hook the FIRST Y-Spliter into the Onkyo805's Center Pre-Out. This will now be TWO center pre-outs. Hook the SECOND Y-Spliter's Male to one of the FIRST Y-Spliter's Female. Now you have THREE Center Pre-Outs. Connect the 1st & 2nd center pre-outs to a 2-channel external amp. Connect the ext amp to the HIGH & MID speaker terminals of the CLR3000. Using a subwoofer cable, Coaxial Digital RCA cable, or just plain RCA cable, hook the 3rd center pre-out to the CLR3000's CENTER FULL RANGE RCA input.
Now you have TRI-AMP the Center speaker and BI-AMP the Front L/R & Surround Speakers!
P.S. Since your Onkyo 805 is already a great receiver, I don't think tri-amping the CLR3000 will make a difference. I'm Tri-amping my CLR3000 with 3 channels of 300 wpc (4 ohms), but I sure can't tell a lick of difference!
P.P.S. Since the impedance of your DefTechs range from 2-8 ohms (Home Theater Magazine Lab), you are probably pumping 160-watts (6 ohms) per channel into each speaker even when you don't use an external amp!