If you want a HPA-21, buy a First Watt F1J instead and save some cash while also receiving something that can double as a heater for your listening room ;)
No offence to Bakoon but not being able to trickle charge while listening is beyond stupid design choice. Price is also very high given other options at that range.
Have you listened to HPA-21? I just suggested one of the best amps, if not the best, you can get that also fits his price range. If you want a "wire with gain" sound (or lack of sound) of course. If you want a colored sound you have plenty of other amps to choose from.
I agree that it is annoying if you forget to recharge the batteries, but if you listen to headphones more than 6½ hours without a break I think you will soon have other problems...
I care more about the sound than the comfort. Otherwise it's like saying "that red Italian car doesn't have a back seat, that's beyond stupid design choice".