Powered sub peaks

[MichaelJHuman]

I got a 10" Dayton powered sub kit from Parts Express. I assembled it, and have been running some test tones into it. (Found a woofer problem in my ACI Sapphire doing this, which ACI promptly fixed!)

I have the standard RS SPL meter. C weighted, set to slow etc. I notice a pretty big response peak from 90hz to 100hz or so. I want 70 dB, but I get 80 dB.

I shut off the mains and the it dropped a few dB. I have a high pass filter in my Yamaha's pre/main loop, so that makes sense.

Is this 90 dB peak probably caused by the room acoustics? I can't really move things around, because this system is in my bedroom and placement is pretty much mandated.