I would suggest 552 on top with Nd555 underneath. Then spare shelf then 500 head unit. 552 likes to be on top and give 500 likes some space above and below.
Try it out and see if it makes a difference. It may not!
I hear you. It seared my soul to burn two bases to give some air under the units. If I donβt put my LP12 on the top shelf. I will probably bite me tongue and do it.