Thanks guys for all your input!

Mark: Good to hear these things are effectively external soundcards. Because that's what I'm after. Combined with the ease of having the volume button in front of me, and also the headphones jack.
I've just come to realize in the last couple of weeks how much hiss there is to hear with my internal sound card. And to think I paid extra for this motherboard because this one featured a Creative Labs (as opposed to Realtek) audio chipset and some shielding. Not enough obviously.

Tony: I don't, never have and probably never will do any audio production. I do enjoy good quality sound though. And if I may believe a lot of the reviews I've read on these things, they *do* seem able to improve the sound of sound files quite a but, even of lossless ones. That combined with the idea that all hiss will be gone is very tempting to me.
With regards to my speakers: these are already pretty good. They are large standing speakers about 3 feet tall (that's also the main reason why I would like to replace them with the LS50's : smaller footprint for an equally good -probably even better- sound quality)
The Bluetooth feature on the DAC is indeed to allow portable devices to easily connect to them and play their music. I'm not particularly fond of Bluetooth and would always prefer a cable given the chance, but it's also one of those "nice to have" features. The true connection to the PC is indeed via USB.

Dan : Thanks for your input! I must say it eases my mind to see you are driving even bigger speakers that I own with such a little amp, and doing so without a problem!
The Teac model you own would have even been my first choice, but unfortunately it's discontinued. I believe the AI-301DA-BK model I linked to is its successor, but I'm not sure.

To be honest: I'm in doubt between the Teac and the Denon. It's strange to see: in the US, it seems the Denon costs $599 and the Teac $335. If I was living in the US, that would make it an easy choice. But here in Europe, the Denon costs €409 and the Teac €457!
Also strange: when looking at the US and EU spec sheets of the Teac, they are identical, apart from the one line talking about the power of the amp. On the US sheet it says 60W + 60W (4 Ohm), on the EU sheet it says 40W + 40W (4 Ohm). I don't think this is a small difference. I think I'll give Teac support a call tomorrow, see what gives. If this is for real, and the thing will accept voltages from 110v up to 240v, it might be worth it just buying it in the US (or maybe the UK, but it seems Amazon UK doesn't sell it - other shops do, starting at £319, also only 40W + 40W though )