Regarding SQ improvements with external dac - I think you'll notice the difference, but how big it'll be - it'll be up to you to judge that
The biggest gain will be imo from better headphone amp than one in your external sb.
Yep to all... the built in amp will definitely be the weak link there.
To the OP it's not that they're bad speakers - but there definitely are much, much better ones. Kind of like an Internet connection - you can view 90-95% of all web content over dial-up... at least until you get a broadband connection. Then it doesn't seem like it anymore.
I've had several mid to high-end (for computer speakers) setups over the past 20 years - the best being an Advent 2.1 setup. And until I heard PC sound on a real amp, through real speakers... I was pleased with the sound from most of them.
Just stop by your local music pro store (Musician's Friend, Guitar Center, etc..) and listen to some of the powered nearfield monitors there. If you have an iPod - at home plug it in to the analog input on the ADA885's and listen to a selection... then play the same thing through the others at the store.
As an aside - you're not the only one that gives these speakers a glowing recommendation. Found
this high up on the hits in a search.
Potentially they are everything you could want as far as the sound is concerned... neither of us (BSA & myself) can hear what your ears are hearing. However, bear in mind that much of the subjective analysis of sound comes from experience. I still feel that I would need to audition about 1000 more pairs of speakers before I would feel confident in attempting a relatively objective analysis of a speaker based solely on an audition of it. And even then I would defer to measured results under almost every circumstance - other than that of the subjective... "personal preferrence".
However, the great thing is that you only need to hear one set of speakers to know if it sounds good to you or not. The problem comes when you are not necessarily 100% contented with that sound... which assumedly you are not since you are asking what can be done to improve the sound.
The bottom line is that all the expensive electronics in the world - will not appreciably change the sound you are hearing through the same set of speakers (assuming that you are supplying them with a reasonably clean signal in the first place... which you most definitely are if you are using the SPDIF input).
If you love the way they sound completely... then just listen and enjoy the fact that you've already found audio-nirvana... on the other hand, if you're only moderately satisfied with the sound - then the speakers themselves are the first place to make a change. IMHO at least.