[HiFiChris]

Personally, I would either use the DX90 on its own or keep the X5k with a gain-reduced UHA-6S.MKII (benefit of more storage and better battery life) or get a DX80 with a gain-reduced UHA-6S.MKII (benefit of more storage, better battery life and still an organic and analogue sound over the Line Out compared to many other devices' LO).

As for the choice of OP-Amps, I opted for the standard option (both with the original 6S and the MKII) and didn't regret the decision at all.

 

[zg1984]

How dumb do I feel? Well, quite dumb .

I tried the DX90 and essentially heard the same amount of hiss. Did some A/B test versus the Fiio X5ii and still couldn't set these two apart hiss-wise. It was then that I thought it might be the recording after all. Went back to your first message and created a silent audio file, played it on my X5 and didn't hear any hiss whatsoever. When getting the volume almost to max you can hear hissing, but comparing that to the DX90 once more, both give the same hiss.

So if the DX90 is hiss-free, so is the X5ii. Sorry for all the trouble throughout this thread, but I learned a lot from your explanation. Another thing I've learned is that (almost) all recorded music hisses and that I probably have sensitive ears... Regardless, I'm planning to get the amp we've spoken of. Thanks a lot Chris!

 

[HiFiChris]

Hehe, so it was indeed the recording's hiss and not the player's. 

 
Imho it isn't necessary then to get an additional amp, but it is up to you to try.