Unconventional Subwoofer Configuration
I have a Marantz SR6004 A/V receiver with a 5.1 speaker system, including Focal Chorus 826 V speakers for the front left and right, Chorus CC 800 V for the center channel, Chorus 806 V speakers for the surrounds, and Chorus SW 800 V subwoofer. In the receiver, I set the front left and right as Large and the others as Small. I set the subwoofer output to Off and use the receiver's Speaker C function to drive the sub's high-level inputs from the receiver's surround-back speaker outputs with the sub's internal crossover set to 60Hz. This configuration produces a very nice sound; I feel that the bass is more structured and integrated than when I use the crossover in the receiver. My question is, does the receiver lose power in this configuration?
The SR6004's Speaker C setting is designed to let you bi-amp the front speakers using the main and surround-back outputs. For each channel, both outputs send the same full-range audio signal, and the bi-amped speaker must have an internal crossover to direct the highs and lows to the appropriate drivers. In your case, the subwoofer's internal crossover allows only the low frequencies to reach its driver.
To answer your question, the receiver does lose a bit of power when all seven channels are driven. Our measurements reveal that the SR6004 reaches 0.1% distortion at 73.1 watts with five channels driving 8-ohm loads and 1% distortion at 86.4W; with seven channels driving 8 ohms, it reaches 0.1% distortion at 70.8W and 1% distortion at 81.9W. More importantly, in your configuration, the sub is not receiving the LFE channel (the ".1" in 5.1), so you're not hearing the low-frequency effects in movie soundtracks.
You say you prefer the sound of the sub's crossover, but your configuration is far from optimal in other ways, so I recommend that you reconsider your strategy. I would definitely set the front left and right speakers to Small in the AVR and connect its subwoofer output to the sub's LFE input. This bypasses the sub's crossover, so use the receiver's crossover to redirect frequencies below 80Hz (or 60Hz if you prefer) to the sub.
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