But how are you meant to compare? Did you use a SPL meter?
I'll show you what happens when I try to compare them;
Using the same amp for both HD650 and DT880, I'll listen to the HD650, then plug in the DT880 and listen to that. The bass sounds quieter on the DT880 - but I realize, actually, the whole sound is quieter because the DT880 needs more power from the amp to reach a given volume. Then I turn up the volume on the DT880 until the bass sounds as loud as I remembered it on the HD650. Then I switch back to the HD650 - I notice the bass is louder, but then again the whole sound is louder because the HD650 are more efficiently driven from the amp. So I turn it down until the bass sounds as loud as the DT880. And I go round in circles. So I can never make a direct comparison.
So I was wondering if you used an SPL meter when comparing? And if you didn't use an SPL - how are you confident that the HD650 has fuller bass than the DT880? I'm not trying to accuse you of anything - just genuinely interested in how one is meant to make a comparison.
They do require different volume, yes. I just adjust it when it feels comfortable to my ears, no precise matching. HD650 sounds fuller perhaps because of the midbass hump