I was finally able to do my DE aquat workout today. I've been sicker than a mofo. I decided to do a few doubles without the box as I hadn't squated without the box. I put on 225 and was able to get past parallel for a double and had some good speed out of the hole. I think I need to work on my flexibility though as I had to lean forward a bit, lie in a gm, to get to parallel. Going to work harder on strengthening my abs and I need to get a belt.
The whole point of the DE exercise with the box is to break the concentric/eccentric chain by relaxing the hip flexor. This is how you develop the explosive start. You can't do this without the box, you're basically just doing pause squats if you don't relax the psoas and it is impossible without a box.
If you have to lean over that far I would say hamstring strength is more of an issue for you than flexibility.
Actually leaning forward is a often a sign of tight hip flexors. Stretch those psoas and you should notice an immediate improvement.
I did the doubles after I had completed my DE squats on the box.