DH and I attended a concert at HOB last month. It was general admission, so I don't understand what the "tiers" are, unless they refer to the different levels of the club (there are 3 levels).
There is virtually no place to sit at the HOB--it's basically just like a big lounge with everybody standing around. There are a few chairs at the edges of the different levels, but 95% of concertgoers can expect to stand. You can get priority admission by eating at HOB the evening of the concert and purchasing at least one entree per ticket, OR by spending at least $35.00 (per ticket) in the HOB store. This, of course, is all in addition to the ticket price.
However, I'm not sure priority access means you will get a seat--I think it just means you get front-of-the-line priveleges (you have to wait in a l-o-n-g line to get in to concerts). If you are interested in Priority access to the concert, you might want to call HOB and ask about it.