The STF-1 will play higher than 90Hz with the crossover bypassed. For some confirmation of this, check out the LFM-2's specifications: frequency response is listed as being 25Hz to 180Hz (as you've noticed, the two subs are cousins and the STF-1 should approach the same upper limit). Setting the 970's front crossover at 100Hz and disabling the crossover in the sub is the best approach.