You might also want to take the tube cages off the GS150 and let those beauties breath a little. I don't think you have any problem with the amp ventilation, but with only the thin shelf above the shelf will get warm and likely lead to extra heating on your pre. Still, several inches above each is plenty and you have the sides and back open. I would take off the plastic top since your pre is not on the top shelf. You might want to put some StillPoints or other under your amp and pre, that will also add some more air flow from beneath.
My Ref75 has the top off, so it gets air from the sides and back as well, and the power supply of my Ref10 has the top off and I have much less room. Does it get a little toasty, sure, but I've not had any problems.
