I have a general question, since I am quite new to Hifi I don't understand the following. The frequency range of DSD is up to 100kHz, my headphones are up to 45 kHz. But the Lotoo can provide only a frequency range up to 20 kHz. Let put the whole discussion aside whether you can hear something over 20 kHz and let assume that you can, how much sense does it make to use DSD files if the player can output only up to 20 kHz? Does my question make any sense at all or am I misunderstanding something?
There's a difference between the sample rate of DSD (which can go up to MHz) and the frequency range of your hearing or a pair of headphones. You are correct in saying that the human ear can only hear up to about 20kHz; realistically, 18-19kHz for a normal, healthy adult.
Now, the sample rate of DSD, or FLAC, or any other file type has nothing
to do with that. The Hz in sample rate refers to how many times per second the computer checks the waveform for a data point. You can think of it like frames-per-second on a GPU or the refresh rate of a monitor (which is also measured in Hz). So, for example, a DSD256 file with an 11.2MHz sample rate will create a waveform with 11.2 million data points per second. Whereas, a CD rip with a sample rate of 44.1kHz will only obtain 44,100 data points per second, and any data point in between will be estimated by the DAC that's processing it; similar to, again, modern GPU technology, where GPUs can now fill in gaps between frames by "guessing" what they'd look like in order to make the gameplay look smoother or more seamless.
It's really difficult to explain without using graphics and stuff, but the key thing to understand is that sample rate and frequency range are two completely different things. It's, sadly, often misunderstood, but it's not something you should worry about at all in this scenario.