I use 2 aerobic steps. The steps in my gym have attachments that give you 3 levels, which I refer to as high medium(or just "middle"), and low. This generally gives us as much adjustment as we need. 2 steps at the low level measures 11.5 inches (i'm sad enough to have measured).
I must need to work on my stretch reflex because my box squat is almost as high as my free squat. In fact I hit 225 on the box squat first.
My theory/ramblimgs/understanding on it is the following... (open for anyone to pick holes in what I say)
The stretch reflex is an "ability" or a skill. I'm not sure it can be improved directly although I have came across some techniques involving a bounce that apparently improves it directly, I just don't understand this yet. I think your stretch reflex doesn't improve per se, you just get better at using it. Technically increased strength in the muscle should yield a stronger contraction both in general and when the stretch reflex kicks in (which is just a reflexive contraction of the muscle) but, you still need to be efficient at actually using it to be able take advantage.
I tend to think of muscles like elastic bands. Going into a squat, I think about my knees being anchored in place and my hips stretching back like an elastic band being pulled (Like a sling shot, with the hips being the little pouch you place your ammo of choice in), aiming to "snap back" to the starting position. So, i think it's important to "pull" yourself into the squat rather than "lower yourself" down, so you can get maximum snappyness out of the hole. This is also why we should "row" the BB to our chest when we bench.
Once you reach depth, you then have reversal of the bar path. This is something most never give much thought about (and i never until recently), and I think this is key. You want to change direction as fast as possible, almost "violently", if that makes sense. As soon as you have stretched to depth, you instantly and explosively move the opposite way. If you even try this with an empty bar, it can be difficult and really call you out on form/tightness, too.
Technically, the Box Squat trains the opposite. You hit depth and pause, muster up all your might, and blast up. You have an intended break between eccentric and concentric which is the opposite to taking advantage of the stretch reflex. By NOT taking advantage of the stretch reflex, you are just muscling it up, which in turn can make your muscles stronger, which in theory should open up potential for a better stretch reflex. However, the skill still needs to be honed. If you get too used to that break between eccentric and concentric, you'll lose the necessary skill (and mindset) for a quick change in direction when free squatting, and it'll almost feel more difficult because you don't have that Box to sit way back on and pause. Instead you essentially "hover" before blasting up.
You could also say that by taking advantage of the stretch reflex, you get to lift more weight and this will contribute to strength of the muscle, too. I think it probably works both ways. I think the answer is somewhere in the middle and the devil is in the details.
Excuse the rambling, i'm really just thinking out loud, leaving it open for criticism.