Some people ears associate distortion as being Loud, then by turning down the volume a couple of db’s gives the effect of being greatly reduced in volume, but in fact its just the ear determine the difference between a clean and distorted sound, with the system reaching its limitations (i.e. Squaring the waveform).
You may think I’m waffling on a bit here, but the real point I’m try to make is that unlike most things today variable, Audio modules are not designed for Plug and Play without consequence results.
If you really want to do the Head Unit to Bose thing properly, you going to need the following.
Dual Beam Oscilloscope, a decent 4 Ohm load, a quality Volt and Impedance meter, and of course a Bose set up and the Target Head Unit. That way you can measure the Peak or Peak to Peak levels going into 4 Ohms, and with some simple maths, calculate the RMS volt levels (0.7071 DC levels at that impedance) and determine the correct audio and impedance levels when then inputting the HU to the Bose Amp, then again measure the New HU into 4 Ohms, then make the necessary wire wound resistor array to suit the Donner Head Unit.
I will firstly say that everything I know about electronics is self taught from guess / experimenting / reading stuff on the web. 
Now, I would be interested to actually carry out them tests. I’m new to oscilloscopes. Is a Dual Beam Oscilloscope the same as a dual trace? If it is I’ve got two collecting dust, I haven't got round to using yet, so this sounds like a good excuse to dig them out! 
If you used wire wound resistors, would they act as a low pass filter? Just thinking about them acting like a coil/inductor? Again, if I’m honest, I’ve know idea if it would have this effect or not, just a question really!
Now I know impedance matching is important on the output stage (esp when output transformers are used like in valve amps), but I didn’t know it was as important on the input to an amp?
I also didn’t realise that the headunit could potentially be seeing a dead short when powering an amp! I always assumed it would be the other way round, and that if anything the amp wouldn't have enough of load to drive :-/
Look forward to reading peoples opinions / answers on this topic! 
Hi Ian, if you really are interested in doing some checks on this, then PM me and I'll try and do a guide for it (don't really want to bore people on here with the details).
Dual Beam or Dual Trace Scopes, meaning 2 Scope inputs, just makes it easier for input and output measurements at the same time.
I mentioned wire wound resistors coz I'm assuming that the Bose HU output levels are going to be a couple of Watts Rms?, so the Wire Wound Resistor bit (Not a Coil) is purely for heat dissipation reasons. They will have little or no effect at Audio frequencies.
You’re also going to need a constant sound reference, i.e., an Audio sound test CD or Cassette, so you that you’re working within the correct Dynamic Range and the Head Room of the HU, I use to find Dolby Test Cassettes were good for this sort of stuff (400Hz at 0 db referenced across the entire Tape rather than tracks).
As Dave say's a good source of learning is the old local Library, paying attention to Ohms Law (VIR) and Power (PVA) is a good place to start, and in particular, Scope measurements, i.e., how to calculate, Modulation, Voltage, Frequencies, effects across loads etc, and calculating RMS level of waveforms (Root Mean Square, its the bit that if you was to flatten a Peak variable waveform into its DC equivalent), all good stuff and interesting, assuming that’s your Cup of Tea of course.
Regards Chris.
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Topic: aftermarket stereo with bose (Read 5802 times)