I think reaching the valves is the most important consideration and all other rig adjustments should happen around that. You mentioned you dropped your tanks. I think you should drop your bands to RAISE the tanks relative to your body. Raising the tanks will allow you to access the valves easier.
I dive 7 mm neo in cold water (tomorrow will be 38 degrees) so I wear lots of garments to stay warm. I had to adjust my bands a few times to "dial it in" such that I could grab my valves every time. Forget stretching exercises and changing your suit or closing your drysuit valve to do a valve drill. We got seconds to save our air supply and that's all that matters. (Well to clarify, stretching exercises are fine but fix the problem now, then perhaps with time and stretching exercises you can lower your tanks if that is important to you).
I had another thought...do you put your thumb through a slot on your under garments to hold them in place? I have long arms and if I did this I would have restricted reaching ability.
I see a lot of double tank divers who would have no prayer of reaching their valves. I think there may be some bizarre diving pressure to wear your tanks low (sort of like a guitar player looks "more cool" if they carry the guitar really low). Ha ha, anyway, raise the tanks to increase access and make sure you are not restricted by your undergarments.
--Matt
