Splitting audio with Y-cable reduce quality?

Well, with a splitter you will be reducing the power to your HD558's, so you may notice some slight loss, I think.

I would say, the cheapest solution is to get the splitter anyway, but disconnect your speakers from the splitter when you want to use your 'phones.

Alternatively, I'd say you should save up for a ZERO or FUN with which to get even better sound and have both headphones and speakers plugged in simultaneously - with a push of a button, you switch between headphone listening and speakers.  Quite a bit more expensive than just a simple splitter. 

Also replaces your Xonar.

Splitters, on the other hand, are a dime a dozen, so wearing one out isn't a big deal.  A well-made one will take plenty of punishment over years, I think, though I wouldn't know for sure since I don't use splitters nor in the way you will.

Not sure what to recommend, I haven't seen "audiophile-quality" splitters.

Try to find something like this locally.

The Tiangyung ZERO amp/DAC or Audio-gd FUN amp/DAC.  Search for more on them, there's plenty of posts and threads on them here.

I think you are better off plugging in your headphones to your soundcard than your speakers' headphone output.

If you go for the E5, connect the E5 after the splitter.  Your headphones will want the extra juice, not the speakers.

I have a similar setup to what you describe. My sound card has a splitter going out to my receiver and to my MS2s. At first I was worried about using a splitter but now I have been using it for a couple months and may be able to help you.

I determined that the splitter does have an effect on the sound. I'm not sure a bout the sound quality itself but the power going to the headphones is definitely suffering. Try this to see for yourself, while listening to your headphones unplug the speakers from the splitter and see if they get louder or even sound better. That's what I did and the volume difference was obvious. I don't like using the splitter but I don't have many options for now so after listening closely with and without the splitter and not noticing any sound quality differences it is back on and probably for good. 

I hope I was some help.

The only problem you may introduce if the cable used in the splitter isn't the best quality it may degrade the sound slightly,  for the majority of set ups howevery this wont be noticeable.

I used to split the front out from my DX one going to my headphones the other to my home surround receiver Line In. It worked OK, but was always a stop gap until I got a dedicated network media player.

As previously suggested  multi input headphone amp which has a switch on the front is the ideal solution, as you will get convenience and better sound from your headphones.