your Android onboard Audio is crap. At least it is not ideal compared to a audio spezialised DAP, for example. You will need to bypass the internal Android audio system, if you want to use a external DAC or DAC/Amp.
USB Audio Player Pro for Android devices - is around 7(add another 5 to unlock mqa) bucks and very usefull/ for me without alternatives, if I want to use a Android device as source for my own FLACs, Streaming Tidal(full mqa unfold with ext. DAC), Qobuz(dark mode) or online radio like radioparadise.
since years, there are updates on a weekly basis and include new services, new hardware or bugfixes
Koss KSC 75s or Porta Pro - both with Yaxi pads - both stay under 50 bucks and are a good start
The 75s are a bit wider and airy while the Porta Pro is still wide and a touch warm
The 75s drivers are different vs the PP and provide a higher resolution
for a sub 100 Bluetooth dongle I can recommend the Earstudio ES 100 and the Fiio BTR3
while the ES100 is more power, provides even balanced hf out and has physical buttons for all controls and a fantastic app, which got many updates(firmware and functions) the BTR3 pairs better with brighter sounding headphones and IEMs.
The Sennheiser Massdrop collaboration brought us the 58x, which is easy to drive and does not change much with the source - so it´s a sub 200 solution for these without a DAP, DAC/Amp
it does not scale as much as the legendary HD650 or 600
my office combination is simple: ifi xDSD > ifi OTG cable > custom balanced cable > Focal Elegia - out of phone or tablet with USB Audio Player Pro Android App(Tidal mqa unfold and Qobuz dark mode, …)
it´s sub 1k, if you are lucky, like me and the synergy is perfect for my preferences
you get a nice Burr Brown implementation, with enough power for most modern easy to drive over Ear headphones
the sound option xBASS+, which is analog and no dsp, is very subtle and helps the Elegia with some songs. It brings the Elegia maybe a touch closer to the Dan Clark Audio AOC2 in the low end.
I did audition the DCA and did use the xDSD with/without the xBASS+ on both headphones.
the Elegia is hard to beat for long time comfort and sound + quite open for a closed back
the xDSD brings also the best Bluetooth implementation I know(tried the Fiio Q5) and the optical/coax digital in as added DAC inputs to the USB in
the synergy between Sennheiser HD650/6xx and the ifi xDSD is not there - what I had to learn over the last days
one could add the ifi xCAN for way more power and the new xBASS+ 2 option.
So you could choose to add Bass, push the mids presence or both.
the synergy between Beyerdynamic DT 1990s (analytical pads) and the ifi xDSD is phenomenal, for my preferences with some music. I know it´s not really respected among most audiophiles, though I love it from time to time on shorter listening sessions
the last but not least, is the Tin Hifi T4 IEM with balanced cable out of the ifi xDSD is very good synergy wise and again the xBASS+ helps the T4 with some songs - here you have a top synergy under 500 bucks
RME ADI 2 DAC fs (4493) > THX AAA 789 > balanced cable > HD 650/6xx - synergy is good enough
this combination won so mutch more in many ways over the ifi xDSD.
I am not used to the 6xx yet. It´s neutral warm represantion was to thick out of the xDSD.
I am going to edit this part after some more time with the 6xx
RME ADI 2 DAC fs (4493) > THX AAA 789 > custom balanced cable > custom balanced adapter > Focal Elegia
the Synergy is great and I miss absolutely nothing - this is used on the sofa, on most evenings for a few hours, while cuddling with wife and the dog - prefer not to use a open back in a seperate listening room.
I have linked some official threads and reviews in the text
The best review I re-read was this one: iFi Audio xDSD by David @pennstac