I will admit, I'm at a loss for guidance here...
Typically they pile if too cold..not too hot...but you say with a higher temp bulb they die too? If it is a temp issue, my only thought with having it hotter and they die...they didn't have an area to get out of the light? ...if they did, then maybe another issue could be going on...type of feed? amount of feed? Knowing where you got the eggs or day olds...do they have healthy birds? Is there enough open ventilation to allow fresh air in? Some times in an all glass brooder like an aquarium, you have a green house effect where even though the light is at one end, the heat cannot escape and the babies die. I think you said you had a plastic tub though
Perhaps someone else will respond with other possible solutions.
I'm sorry to hear about your losses.