You can adjust the clamping force if needed but if you are going to return the HD650 I would not bother.
The output resistance of the amp, being it 0.03 or 3.00 is completely immaterial for the HD600.
The DF number difference is huge but that's just a meaningless number used by sellers of equipment.
What damps is not the output resistance of the amp. It is tempting to think this way as output R and impedance are in parallel.
They are in parallel for voltage but not for current. It's the current that damps.
For current the entire chain is in series: Rout+ Rcable (incl connectors) + Rdc driver.
So the same U (they are in parallel, lets use 1V) will be 0.035 (amp) + 0.3 (cable) 300 = 300.335 ohm = 3.33mA in current
with 3 ohm amp: 3 (amp) + 0.3 (cable) 300 = 303.3 ohm = 3.30mA in current so practical change is nothing.
Note: for low impedance headphones with a substantial impedance peak this becomes an entirely different thing.
Don't worry about hearing differences. There are millions of people stating they hear differences in headphone cables. There are folks spending hundreds of dollars on cables. When you feel better and only spend little money it is totally worth it (as it brings peace of mind)
Let me show you the differences, actually measured
acoustically (so output of the drivers) between a silver cable and a copper cable.
In this case a 250 ohm DT1770 with the
original 3m copper cable vs a
1m braided silver cable.
And in this case the return wire is even a common wire. The HD600/650 cable is 4-wire.
The reason you only see the teal trace is because the purple trace is underneath. No difference.
Also note that there isn't a single cable in the world that acts as band filter or can introduce changes in the audible band.
Well ...there is a cable that actually does but that's something entirely else and totally on purpose