Haven't heard the DX180 or the RU7 myself, so I'm just relying on general consensus of those two devices. But to my ear, I think a lot of what
@o0genesis0o says in their review of the R4 jives with what I'm hearing: flat staging, a little noisy, with subtly dynamic lows. By comparison to the other amps and DACs I've got lying around, I prefer what I hear in dongles at and even below its price.
But IMO that's to be expected for an all-in-one device like the R4 with a screen, a processor, and other costly features. Cuts have to be made somewhere in order to sell it so cheaply. Ultimately, I much prefer the convenience of just carrying around one device with me, so I'm digging it. I personally don't see the value in spending more than $300 for a DAP that's just going to need to be replaced not too far down the road. But that's me.
Nevertheless, sound quality preferences are what they are. I'm a firm believer in
different rather than
better when it comes to audio gear, regardless of price difference. I certainly don't think you're wrong to like it more than other pricier devices. Tastes are opinions, and opinions are like... well, I'm sure you know the rest!