It's impressive that you have the knowledge and skill to DIY mods. If I were you, I'd build myself an M^3, Beta22 or Dynalo for my amp and a y2, or better yet, a Buffalo III for the DAC. It would be a very significant leap from what you're currently using.
I say this because I've owned all the high-end sound cards and put in the best components on them. The upgraded STX was the best of them, but it was smacked around by a lowly Musiland 02US and y2 DAC. The Buffalo was easily in a different league from all of these and then the stuff I owned later kept getting better.
Where am I now? At a point where I cannot even listen to soundcards. You can poke and prod all you want but there are inherent limitations to them. By the time you get it sounding half-decent, you've spent more money and time than would have gone into just buying a better product.
Nothing is perfect. The HD800 certainly isn't and neither is its housing. My point, however, was that it's definitely *not* what's limiting your setup. Leona is a very good material for its purpose and as I said before; if after the major component upgrades you aren't satisfied, you can tackle such mods. Otherwise it's like trying to make a Civic faster by adding a wing to it before upgrading the engine. Focus on what will give you greater gains first.