When you found the clipping point of your radio with the RCAs hooked into the amp but the speaker wires unplugged, were all your filters set off or crossovers set so they don't interfere?
Check any eq setting on your radio, turn them off, see if that makes a difference.
It is possible the radio is the route of all the trouble or it could be an RCA dud. Pull all your RCAs out and check the connections to make sure they are tight and not loose just in case.
Try changing the battery in the DD-1 for a fresh one.
Try the 40hz - 0db track, and see if it gives you any different results.
It is possible that you are seeing expected distortion at 1/2 to 3/4 volume and that is just normal for the radio, not many aftermarket radios can go all the way up without distortion.
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You mentioned you were looking at the DEH 80 PRS, this is the current radio I am using and although I haven't measured the voltage of my RCAs, the radios limit is 62 but with volume source input +1 (bluetooth Iphone) I don't clip until 62, so I tuned at 61.
Only issues I have with this radio is the screen is not very bright during the day, at night it is no problem. Also it has two modes network and standard - I have only used network mode since I am using an active setup so all my crossovers are within the radio instead of on my amps.
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Btw nice wire job, looks clean as hell man!