Do you need a POWERFUL amp?
1. Here's how power is computed. These are laws of physics not subject to audiophilia.
Current = Voltage / Impedance
Power = Current ^ 2 * Impedance.
Nothing about the HD-650's input impedance (nominally 300 Ohms but at some frequencies ~500 Ohms) or its voltage (or current requirement) is special in any way for a full-sized headphone.
2. There is a rule (guideline) that the headphones' input impedance should be at least 8 times the output impedance for proper damping.
It's easy to find an amp whose output impedance is less then 300 / 8 = 37.5 Ohms.
3. Does the HD-650 sound the same with every amplifier?
Absolutely NOT Amplifiers all have distortion (e.g., THD, IM) and a noise floor. Tube amplifiers tend to have different harmonic distortion that solid-state amps because that's the way tubes work..
In conclusion, the next time you read claim about a requirement for a POWERFUL amp, ask why. If the response is "pairing", "synergy", or "scaling", you've received a non-answer and can safely move along. Which amplifier sounds best with the HD-650? It's nothing more than personal preference not POWERFULNESS.