Originally Posted by Fountainhead

4 Volts is at maximum, or nominal, BAF? You know that at nominal to low listening level the head unit isn't putting 4V out right? 4 V is maximum level. I think you may be confusing impedance with voltage? Voltage varies with level and impedance varies with frequency. In the case of line outputs usually the output op amps are low impedance 600 to 1000 ohms, and the inputs of the power amp are usually high impedance up to 10K typically. Those values are typically chosen because the designer considers the long runs of cable needing a low impedance to drive them to reduce the losses and the resistance to the capacitance contained in the system cabling to affect high frequency response. Resistance value doesn't change with frequency so that's why they are used to terminate op amp inputs and outputs.

in this case a lot of noise high frequency sounds like mismatched gains. I would mute the head unit output and see if the hiss lessens-it's a "divide and conquer" scenario. If one really has to one could build a voltage divider to drop head unit nominal output by whatever amount you need to. Divide the level, drop the hiss, etc., to match what the BOSE amp wants to see. I can't help but think 1V RMS is enough to drive it to clipping...