After an evening of recalibrating and EQing everything, and playing with both sets of subs, there is a noticeable tonal difference between the two when played separately. The Avalanche subs have a heavier, richer sound. The HT18s have a lighter, higher frequency sound when playing the same wide band bass test tone.
The different drivers, enclosure materials, enclosure sizes, driver orientations, and age of the drivers are all contributing im sure. The HT18s have been sitting in boxes for a few years and the suspensions arent loosened up at all, so the tone may change over time. When all four are played together, it sounds great, one set compliments the other. If i could only pick one set to use though, it would be the Avalanche sonosubs for sure.