Thanks Kevin for taking the time to provide these parameters, they are really valuable in setting up my Home Theater system.
For my system, here is the signal chain:
DVD audio track (16 bits) --> Windows Media Player --> Xonar Deluxe 3.1 Audio Card --> Crown xti-2000 --> Klipsch K-Horns
First, the max output level of the audio card is 2.2 vrms, which would overdrive the input of the amp with no attenuation. Using your figures above for the attenuator I would want to bring it down 4 notches, -4.5 dB to keep the max output from the sound card over driving the input to the amp.
Next, at this max level, the output from the amp is approx 62 vrms which is too much for the K-Horns. The sustained level for the K-Horns is 100w (28 volts), peak is 400w (56 volts). So as a minimum, the max output of the amp cannot exceed 56 volts .. so I would limit the input to the amp by another notch which would bring the max output voltage down from 62 volts to 52 volts.
Given that the music level cannot be any greater than +/- 32765 (15 bits) in the source file, the maximum output signal from the sound card will be 2.2 vrms which is the input to the attenuator. With the attenuator down 5 notches, or -6dB, the output voltage will be 52 volts .. or about 338 watts (assuming 8 ohm speaker). This will result in an SPL of 124dB which is a bit excessive for a home theater.
THX recommends an ave listening level of 85dB with a 20dB headroom, so max would be 105dB, which is 1 watt for the K-Horn or 2.83 volts. So to make sure that max output from the source material did not exceed 105dB out of the speaker, I would need to attenuate -25dB .. or, rotate the attenuator clockwise from full attenuation to about notch 6.
I tried this and that seems to work quite well. At max output from the sound card, the speakers are loud but not damaging. I have a calibrated mic and will attempt at measuring SPL to confirm all this.