Nice, man!!! Congrats. That surprises me because I've heard a lot of people complain about sub-bass distortion at high volumes with the DT770. My DT770 Premium 600 ohm definitely has it, its the only reason they aren't my favorite cans because other than that I love them and I particularly love their well extended, detailed, thumping bass. Do those cups allow more airflow than the stock cups? The sub-bass distortion in mine seems to be related to not having quite enough air flow to allow the drivers to push the air needed for really low sub-bass notes when the volume is cranked up really high.
Did you see the pictures of my deep cup Darths? You'd think with the added volume of the cups it helps. I think with the depth of the cups, the modifications he does to the drivers (which we don't know what they are), these things can really go deep. No distortion being fed by my vintage Pioneer amps.