Looks for Headphone Tube Amp

Congratulations. One can get a rewarding system at most investment levels by carefully chosen components. Since some of your stuff is still new, it should sound a bit better after passing the 200+ hours on everything.

My experience is portable are generally not nearly as good as desktop, I have tried a few.  I definitely include my own A&K Ultimate SP1000 (~ $3.5K) with attached extra amp. It is good, but not nearly as good as a similar desktop equipment.

You have opportunities to get substantial improvements from upgrading the amp, DAC, and streamer (source). 

About fifteen years ago I bought a decent set of headphones for my iPod… then did incremental upgrades (each a step up) to my system….  7 different amps, 5 DACs, and 5 different streamers. You can see my current system by clicking on my user name. Each of the many upgrades was substantial and wonderfully rewarding. The more effort and money you invest can be very enjoyable. I think amp, DAC, and streamer in that order is the way to go until they have similar quality (think very roughly cost). Then do it again. When your system is getting really good, do not neglect the streamer… it is easy to think the streamer just forwards bits… how could that impact the sound… it does. I learned by delaying too long.

I’d have a look at Schiit, the Vali 2 although the Valhalla is above your budget, but depending on your DAC and streamer may be worth considering.