Been back in the hobby for a few years now. Taking small steps to try to upgrade incrementally. Room sounds very good with all the major stuff done for now. Still have some untreated areas in the room that I will get to soon....for now it is time to enjoy.
I do know for certain that soft wood pine with its open grain structure is a poor choice and will soft and defuse the sound a fair amount. My experience is oak really boosts the bass. Maple is also a hard wood like oak with a dense grain and is supposed to be a good choice, but it just didn't work for me, but many swear by it.
Very fine audiophile equipment and listening room. It is excellent that you chose to add an amp w/ extra speakers for the kitchen with high-end equipment.
Is you diy rack maple. I had my tube integrated amp on a 2" thick maple amp stand with solid aluminum feet set on the floor, before I put it on the equipment rack of laminate veneered mdf. The difference was very noticeable. I always read maple is a premier choice for equipment, but I found it softened the sound by a large amount, removed detail, and eliminated dynamics and the leading edge of the sound. Maybe for some equipment it's a good choice but it was not my cup of tea.
Also if your steel uprights went straight to floor w/o being welded to a cross member at the bottom this should tighten up the sound. And if each shelf is set on isolation spikes that would help, as well as filling your hollow steel square stock w/ sand.