I've been reading this over and over trying to make any sence from this. So far, nothing, I'm just getting more and more confused.
If it is JUST the tacho showing high RPM and NOT the engine going to high RPM then I can't see any connection to the timing chain.
It sounds to me that the previous owner was talking out his arse to sell you the car.
As suggested, disconnect the Amp and Sub and see what happens.
Also, does this problem happen all the time?
If its just when the car is cold, your issue could be with the automatic choke (to use old terminology)
2013 Ford Focus ST - Performance Blue, Cobb AP
Gone 2006 3 MPS Sports - True Red, Koni FSD shocks, in dash OEM look Ebay GPS/DVD, Hypertech, RX8 Rims