Play the sub alone and listen to it.
I would be suspect of the settings....any shop that tells you about box rise but forgets to tell you about box fall is confused. He is just regurgitating something he doesn't understand. To explain, all drivers have an impedance response and this response can change based on the environment it is mounted in. This response is a curve, it does not only rise but also falls at differing frequencies. Subs are specified in thermal and mechanical power handling to deal with both the response of the driver and the box it is in. Having the installer "scare" you that you aren't getting power because of rise is completely irrelevant. To make matters worse, implying that you would hear a difference in a 1500w amp as compared to a 1000w is asinine.
Go enjoy your setup. Your installers comments take with a grain of salt. Since you have to take them with a grain of salt I personally would double check everything he did.