Most stereo monitors have analog inputs. The only way to accept digital input is to have a receiver/amplifier box for this simple reason: if you had 6 such monitors, how is every one supposed to know which channel to play? It's the receiver's job to split channels. (It could also be the soundcard's.)
Overall, for home theatre and pc surround sound all-in-one makes sense since the signal is expected to be digital and more than 2 channels, but they are not the be all and end all of the speaker world as walking at the Best Buy home theater alley would make you think.
The whole idea with powered bookshelf and monitors is that each have an integrated amplifier, so they can only make use of analog input unless... they come with a receiver box to split the digital signal and send analog sound to each of them. Am I turning in circles here?
Pretty much all studios with powered high-end monitors work on that principle: convert your digital away from the speakers. The sound quality then depends on your digital-to-analog conversion, which is done in your soundcard, or receiver, etc.
Analog input provides several benefits:
- It's not limited to current digital formats (the few digital studio monitors that were marketed in the 90s probably serve as book weights today)
- you can buy a better soundcard with quality higher than consumer-level receiver/amplifier without duplication of components (and money wasted for it)
- you know care was taken in the analog inputs of your speakers
- you can move your speakers easily and plug them to to other things, such as hifi cd players and turntables, super nintendos, ipods and whatever with an output jack you have at your summer bbqs.
- You let many speaker companies do what they do best: speakers. The digital-surround-speakers companies are relatively limited and far from as diverse and interesting as traditional analog monitors/bookshelf speakers.
So.. any downsides?
I think a decent soundcard is in order for 200$+ speakers (even more so for 200$ PER speaker). A Creative card will sound better (clearer) than on-board, and you can check out prosumer stuff such as M-Audio and others if you're so inclined.
Are you planning to want surround sound in the near future? Surround soundcards don't come by the dozen, and neither do non-amplified receivers/splitters.
Unwanted noise from soundcards does exist, however. I've sometimes heard some strange noises when moving my mouse, moving windows and accessing HDDs. These are due to interference and voltage instability due to power supplies, motherboards, wall outlets, shaky components, but can be usually troubleshooted and fixed by changing or moving cables, power supplies, etc.
TL;DR: does analog input sounds worse than digital? No, sound always get converted to analog at some point. You choose where and how.
I've never heard of audioengine. Any good? I would recommend Behringers, M-Audio, KRKs, too.