For me having a DAC/AMP is really no problem, I just want to have a great listening experience.
Do you think there's a big difference in sound quality, connecting your phone via aux to an AMP vs connecting a device with USB to a DAC to an AMP(or DAC/AMP combo)?
Sadly none of your suggested AMPs are available here, so I don't think I can buy them because I also want to be able to get a quick replace product if something is wrong with it.
Here are some Brands that I can choose from:
...
Can you suggest any of these?
Edit: the FiiO Q5 is actually available. But in comparison to the RME ADI-2, is it any good? Because I don't want to buy something mid-range now just to buy something better later.
Hmm... allow me to answer your questions as best I can. Hopefully I will address your concerns even if I'm not directly responding line by line.
I think it's always better to feed an amp a line level output as opposed to maxing out your phone's volume on the headphone jack. To keep the signal clean, minimize distortion, and all that. I know some android devices do analog audio better than others, but I don't know which ones offhand. And I don't know if any output line level to avoid double amping. So, my best guess would be, if you're going to externally amp a phone, might as well offload the DAC as well, since you're probably going to save your phone battery in the long run.
Admittedly, though price tiers can be misleading, the FiiO Q5 does offer a lot of solution for your money, compared to a $1000 DAC/amp desktop combo. First of all, (and this is something I know very little about, but) if the USB input on the back of the ADI-2 won't recognize mobile devices for whatever reason, then you will probably need a bluetooth receiver to connect your phone to the DAC. Minimal, but additional cost and more stuff to potentially break.
The RME ADI-2 seems like a pretty great all-in-one device, on paper though. The real benefits to me are the great specs (though specs only tell part of the story), the DSP, the IEM out, and the power rating. The extra power is nice, but the LCD-2c won't need most of it. And you won't need the IEM out either, for obvious reasons. Does your phone already have a decent digital EQ? Then you won't *need* another one, though the implementation on the ADI-2 is cool and customizable and probably better than any Android DSP. Normally, I think it's easier to achieve better quality through a balanced connection, but the ADI-2 seems to have a fairly excellent spec sheet for a single-ended out, so it will likely sound just as good as the balanced out on the stock Q5, if not a tad better.
Ultimately, I recommend going for something cheaper and more flexible like the FiiO Q5. True, the Q5 out of the box might not be as good as the ADI-2, but with the portability factor (something I would not consider if it were not for the fact that you're using your phone as a digital source), including built-in bluetooth, different amp options (along with a recently announced THX amp module that looks to closely rival the performance of the ADI-2), and a cost that's less than half of the ADI-2, the Q5 seems like a much better deal for a small, upfront hit in performance. Then, you can put any savings into upgrading the Q5 amp module or invest in a nice balanced cable for the LCD-2c and close that gap between the ADI-2 and the Q5 even further.
Something to keep in mind is the update to the Q5 (Q5s) was recently announced, so the Q5 might go on sale, or you can wait for the Q5s to come out and get the better device. Sometimes, spending less money upfront means having more options should you decide that you don't want to feel stuck with some big expensive piece of gear.
