The quickie is not really very powerful - it's a preamp, not a headphone amp. Even with fresh batteries, you may only see 2mW into the AKG's. That's enough to match normal loudspeaker use, and probably OK for late-night listening without disturbing others. But a lot of people run headphones louder that they would speakers. (Perhaps it's because the visceral body response is missing?). In most cases, that ain't going to happen with the Quickie.
You can try changing the cathode resistors from 1K to 2K ohms, which makes a somewhat better power match. Or double the high voltage to 72 volts. Those will buy a few more dB, maybe 1-2dB and 4-6dB respectively.
