51 are amazing for desk or not, but i think its overkill for desk.Lol as an owner of 8351, 41, and 31s I think the 51s are a bit absurd on a desk(and yes ive seen the photo of someone using 8361 at a desk...). If you want to upgrade further past 8331s you should get subs and learn to configure them right or get the GLM Genelec ones. That will do way more for you.
Bonus is that I've seen 8331s heavily discounted as new and for sale used more than any of the other models.
I have tons and tons of reserves for desk use, 31s + genelec 7360 sub would provide same awesome experience for cheaper + easier placement for better bass.