hmm dont use Dbells much if at all
but there is no way i can get inot an arch like on flat bench with using the bar so therefore i would prolly just make sure to get a good upper back arch rather than a lower back arch..
I am on my toes and try to push the hells to the ground..It sounds like you need to move your fett out to creat a more stable base..having your feet close will cause you to wobble..just slide your feet forward a little more,have your heels pointing towards your shoulders