I think you do turn off some entry-level types with a mandatory "donation" that doesn't feel like the ticket price.
As an alternative, you could reduce the number of season tickets available to non-donors. Manufacture some scarcity. "Sorry the 5,000 season tix for non-NCC members are sold out, but if you join the NCC even at our lowest level, then there ARE tickets available at the same price but in a slightly better location".
I know that is kind of semantics, but I think it accomplished the same goal and would be better received. You create the perception of scarcity, and the perception of a benefit for your donation with improved seats (even if it is only 2 rows better in reality). Both those things create value in the mind of the buyer. Even at the same ticket price. You'd be not only getting a donation, but a new NCC member, and those are sticky revenue sources.