Sutton,
After researching what other people are doing and the design of the P-500, I think you can safely run the single 4 Ohm VC Lambda with the Hafler amplifier in bridged mode. I say this because the P-500 is the professional version of the DH-500 and it is more robust with a higher rated transformer and better protection circuitry. Even though the amp is rated at 800 Watts at 8 Ohms in bridged mode, 4 Ohms is safe if the connections are good and tight.
However, there are some caveats: Do not replace the line, rail power (inside the chassis), or speaker fuses with different ratings than the ones called for by Hafler. Also, do not drive the amplifier into clipping (the red lights on the front panel) because the amplifier will sound distorted and damage the speaker. Finally, if the air coming out of the back is very warm and the fan is running at full speed (not half speed), the amp is working too hard and a different speaker load closer to 8 Ohms is called for (obviously, you don't want to block the air intakes on the sides or the vent in the back). At full power (which you should avoid) and 4 Ohm speaker loading there is a potential for massive amounts of power/voltage/current (1600 Watts/80 Volts/20 Amps) at the speaker wires. If they cross, there will be a fire. If they become momentarily disconnected, there will be sparks, if the amp is "on" do not switch the "Stereo/Mono" switch under load, internal parts will fry.
A capacitor in series with the speaker will block the low frequencies depending on the rating. This is not what you want for a subwoofer. What is the preamp source? Is it a real subwoofer output signal?
After all that, if you are still willing, let us know how it works out.
