Rockford specially designed the amps to operate this way....the key was to drop the impedance to almost nothing and the amps took it in stride. Any passive loss was negated. If you didn't use their amps this way, they were nothing but damned expensive two channel amps.
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You cant apply normal theory to this method.
And you cant run electronic crossovers with one amp (these Punch's were two channel amps).....believe me, when Rockford first told us about this we all had the same thoughts. We always used multiple amps with electronic crossovers.
Until they taught us how and we did it.
And cars using that method were winning contests left and right.
After two days at their tech school, we were selling one amp Punch 45, 75 and Punch 150 systems every other day.
I kept a Punch 75 system in my car as a demo....16 speakers off of one stereo amp with nothing but caps and chokes, and it was incredible.
There was a wiring method that ran the subs in mono and the rest in stereo. We kept the entire system wired to 2 ohms or as close as we could get.
If some techy had brought up passive loss listening to it, he would have done so while smiling and having his eardrums beat half to death !