Do you do Reverse Crunches -- that is, crunch-like Leg Lifts? I do a set of regular crunches, then a set of Reverse Crunches, and then I usually feel a bit of a burn in my abs. But on the Leg Lift Crunches I tuck my hands under my lowback just above the butt, so the back is arched; and I fold my legs over each other, trying to focus on not using hips to raise the legs. I don't know if this really addresses your main question about regular crunches, but I do both regular & reverse crunches and these seem to help minimize any lowback pain I have at the time.
As to the question about ab gadgets, I actually think the 6 Second Abs thing actually can work. It has handles on 2 ends, one end you grab with hands, the other end rests on your lap. It has resistance bands in the middle which stretch as you bend over & press the handle ends closer together, so the resistance grows stronger the further you contract. I have one, and it seems fairly effective, but I haven't gotten around to using it very often. But I think I should.
Good luck.