finding positive output lead from preamp

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Hi. ive just purchased a new pa-amp to drive two of my subs. i want to use the preamp from my subwoofer-plateamp. so now i have opened the plate-amp and found the +\- 15V leads to drive the equaliser, rumble filter and individual phase controls im making. and i have found what i belive is the postive lead (one of the leads i will use the connect the pa-amp, the one that connects the preamp and "main" amp), but how can i measure that without breaking anything?
could i send for example a 100hz signal into the input on the preamp, and measure the currency between the lead i think is the ouput lead and the ground?
 
it would be simpler to build a +/- 15v supply for the signal processing. all you need is a 20-0-20V 1A transformer, a small bridge rectifier, 2 4700uf/35V caps and a LM7815/7915 pair. for a few junkbox components you can have clean power for your sub processor.
 
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