All my headphones sound amazing without an external amp actually
Quick list:
Over-The-Ear Headphones:
Meze Classics 99
Ultrasone Dj One Pro
Ultrasone Signature Studio
Ultrasone Signature DXP
MOOH BT Headphones
Beyerdynamic Amiron
IEMs:
Beyerdynamic Xelento
HiFiMAN RE2000
HiFiMAN RE800
Dunu DK-3001
Dunu Falcon-C
Sennheiser IE800
Etymotic ER4-XR
Etymotic ER3-XR
Sennheiser M2 IEBT
Kinera H3
Simgot EN700 Bass
Fiio F5
FiiO F9
FiiO F9Pro
Oriveti New Primacy
Astrotec AM850
iBasso IT01
Mee Audio P2
At this point I'm actually having a blast with using all of the above with AMP5, without adding any external amp.
I do add iDSD Black Label Micro or Nano when using my smartphone or Tablet to watch a video, situation in which they help greately, but AMP5 does an amazing job with the headphones I've tried it with so far (the list above)
Paul said a few pages ago that there is a Cooper and a Titanium version of DX200 incoming, but I don't think he ever announced any other DAP. There is IT04 to look forward to as well from iBasso though.