isolated hollow freq range from sub

I usually just search and read, but I am not finding any answers so I would like to ask a question. Here is what I have: 2007 Tundra Double Cab w/ JBL system, no navigation. Stock HU AudioControl summing unit to bypass the stock amp feeding a 5 channel Alpine PDX amp. Off the amp I am running McIntosh 5.25 component sets in the front doors, Alpine 6.5" coaxials for rear fill in the rear doors. 2 JL 10W1 4ohm single coil subs under the rear seat, prefab down-firing box single chamber. Subs are wired in parallel. Before yesterday everything sounded fantastic. I built the system for good SQ and set up the subs for decent, flat response, clear, complementary bass. Yesterday I was playing a CD I play all the time and the subs hit a note that sounded TERRIBLE. I used the low pass crossover to estimate the frequency. It is a punchy drum note in the 90-100Hz range. The sub is playing above and below this note with good response and volume, but this one note is mildly muted and very hollow. It sounds like there is a hole in the speaker cone or surround. Instead of boom, the note comes out "thunk." It was not doing this previously. I have inspected the box and speakers visually. No damage, no tears, pushing on the sub cone reveals no "feel" difference in the suspension from new. These subs are approx 1 month old. Gains: I set the gains by turning the HU to approx 75% and following the instructions for the AudioControl unit to set just below the unit's clipping "ceiling." I then used the ADD method to set the amplifier gains. I left quite a bit of headroom on the subs as the amp puts out twice their recommended max power and I am not looking to wake up the neighborhood anyway. Any thoughts as to the issue, or the proper approach to troubleshoot this? One other note, as the rest of the US, it has been VERY cold here lately. The subs are doing this when I initially turn on the vehicle, and after they have had some time to warm up. I don't know if this could be an effect of cold air density? Thank you all very much.

I have no idea what your problem could be, but it is not the cold weather. More dense air = better low frequency response. Did you change the crossover settings on your deck or amp?

I believe I have found the problem. The AudioContol unit was clipping badly. I am not sure when but it looks like I must have bumped the adjusting knob on the sub output just enough to push it past its capabilities. This might have something to do with an inconsistent source when I am setting the gain on the AC unit. I try to check that I am OK for CD's, radio, etc, but they play at incredibly different volume levels in relation to the volume number readout on the HU. What is the correct gain setting for CD's is no where close to the correct setting for the radio OR the aux. input. All three are completely different.