Indeed, if you've currently got hiss or hum, moving to an external sound device could potentially improve the situation immensely, as was already discussed.

Hiss or hum can be introduced in a lot of different places in the signal chain, so if you're daisy chaining devices together, such as the audio being produced by your computer and then being sent through a flimsy headphone port down a crappy headphone cable into a cheap splitter adapter plugged into the RCA jacks of a power amplifier which is on a different DC power source than your computer, that's a lot of potential for things to get hissy or groundloopy.

Since those external DACs you listed had their power amps built-in, hopefully the designers made them so that the signal path from the DAC to the speaker jacks was clean, and that they've eliminated any chance for ground loops in their internal signal chain. In that case, it's the shortest and best-controlled possible path between the place where the actual audio signal is generated (the DAC) and the place where it drives voltage to the speakers (the speaker jacks). My expectation is that if you had any hiss or hum at all before, it should be 100 percent gone after you switch to one of these external jobbies, provided it's well-made.

If you don't use the internal power amp built into the external doohickey, though, and instead daisy-chain it into another power amplifier, then you're back to the same old issues again where noise and ground loops start to become possible again.

I still don't think the DAC itself is going to sound any better than what's in your computer. But there's a lot of other stuff those devices can do to make it sound better. Aside from the elimination of noise in the signal chain, they could do additional signal processing and EQ to improve the perceived quality of the sound.