Thanks Tony. Indeed, those are my thoughts too. There's a lot that can go wrong with the sound quality when you're just using analogue out from the PC. An external DAC solves all this. And indeed, the built-in amps seems to be of good quality too. Also the used digital-to-analog converters are better quality components (like BurrBrown's) so that probably doesn't hurt either.

I've just phoned Teac support and they told me that the difference on power wattage between EU and US models is probably because is the US they mention peak output and in the EU they don't. Still strange. They also couldn't confirm to me if the power supply could handle both 110v and 230v, so buying it in the US is probably not a good idea.

Added to that: I just read some posts about the Teac having issues with the USB driver. A lot of people returned the device because of this. This of course is not an issue if you're using the device as a TV amplifier, but for the use I have in mind it's a complete no-go.
So that leaves the Denon and the NAD. The Denon has a higher wattage rating than the NAD does (50W vs 30W per channel), but every review I've read on the NAD says it drives every connected speaker excellently and very loud, so I guess that'll probably be ok. The Denon also seems to break down more easily and have more issues (crackles, pops). NAD warranty and customer service also seems better should something happen. So I guess I'll end up with the NAD D3020.

The only thing that still worries me is that maybe when I change PC's in a couple of years, I won't be able to use the DAC anymore because either there won't be any drivers for the new OS (by then) or USB-A will be gone and USB-C will be the norm. (but that can probably be easily fixed with a converter cable) In other words: that this new amp will have a considerable shorter life span than my old amp had. Even though I do consider 20 years of use to be a formidable success for a medium range amp.