HO Alternator putting out low voltage?

I recently purchased a HO alternator. When i did voltage test it reads 13.0-13.6v at idle with nothing on (stereo, fan, lights, are all off). With everything on it drops to 12.5-12.9v. My alternator is as follows from the test results given to me from where i got it. 

Maximum output                       [A]: 172                           Stator Voltage                           [V]: 5.8

Regulator set point                  [V]: 14.8                          Secondary Regulation            [V]: 14.8

Turn on speed                    [RPM]:1415

Output @ 2000RPM                [A]: 65

Output @ 5000RPM                [A]:162

With everything off when i come to a light my headlights dim. My stock alternator did not do that. All those numbers are at alternator rpm and i was told even at engine idle i will have more output than stock. My question is  the voltage regulator not working properly? When engine is reved the voltage is 13.6-14.2 with no accesories on. If they are it drops will drop with them on.

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1996 Chevy monte carlo LS

2 -12" phoenix gold octane zr

1-rockford fosgate class d 1001bd 1000w rms @ 2 ohm max 2000w

1- 1farad pheonix gold cap

i went there and they ran test as well. Told me to change belt tensioner and belt but still no difference. The shop is about 1 hour away from me, i guess i have to go pay them another visit.