The Coralife Superskimmers have a "silencer" on their airlines. Could something like that be used if the airline is too noisy? It looks a little similar to a bubble counter (just not transparent), basically just a little tube capped on both ends and then two hose nipples on the top, you just attach the airline to one of them. It probably puts a *tiny* amount of restriction on the air but not hugely I would think.
As far as that airline valve goes, I'd totally take it off. Seems to me you want as much air as possible, and compensate by lowering the water level. And if the air bubbles are too big then you just meshmod that sucker!!
Also, the gate valve mods are totally worthwhile, if you can get your hands on a gate valve. One disclaimer, I don't have a Euroreef but do have an ASM (actually gone through a couple ASM's now... when they used to be a cheap alternative to a Euroreef)... I did the gate valve mod on all of them. I found it way easier to work with the skimmer that way (plus it does help with the noise level too).