RE: What Evo to partner an Evo 74 ?
Tough to say how you will percieve 5.9 on the E80. I use it up to 6.3, but sailor weight is part of the equation. I think that if you're reasonably powered up with the 5.9, it will be OK, but for more semi-planing type (x-on) riding, it will not be that good. But on the other hand, for powered up 5.3 sailing the E80 will much more natural than the 90, so its definitely a tough choice.
Considering the other boards you have, the 80 seems like the best choice. It will give you a healthy overlap with the 74 in the critical 4.7-5.3 spectrum so you'll be able to choose boards depending on conditions in this range (not only sail size). In lighter conditions, the E80 will still be super effective on a wave even in light winds, the better the wave the better it works. If there are not much waves and the wind is light onshore so its more B&J riding, you can take your 104. If you're gonna use the board in Sweden, its very, very seldom reasonable waves to ride when in underpowered 5.9 conditions. But if you want the board to excel in superlight wave days in Denmark for example, you might start to look harder on the 90 again.
But again, try to get a test ride on the 80 and see how you feel about it. Email me at firstname.lastname@example.org and we can arrange it (unfortunately I don't have a 90).