At your budget I’d shy away from a tube amp if you’re looking at planar ‘phones. While you can certainly build a very decent headphone rig for $2k, IMO if you can up it closer to $3k you’ll be at a significantly higher level of performance. Also, to keep costs down at least for now and if you have decent WiFi, I’d recommend running a USB cable from your phone or tablet (running Qobuz) to a DAC so it can handle the source/streaming functions until you can justify a better dedicated streamer. So, here is an idea for a pretty excellent headphone setup given what you’re looking for...
- Hifiman Arya open-back planar headphones - $1599
- Singxer SA-1 Class A headphone amp/ preamp - $599
- Musician Pegasus R2R DAC - $1100