If by "slam" you mean sub-bass impact, then you are correct, at least in comparison to some other phones. As shown on the graph below, the HD650's do roll off a bit at the bottom end of the scale, and this perception will be further emphasized by the 650's midbass hump, which makes its sub-bass sound somewhat recessed by comparison.
For this reason, I find that both my HD800's and DT880's do a better job of bringing out sub-bass than my HD650's. However, from a big picture standpoint, I firmly believe the HD650's are better headphones than the 880's (250 ohm), at least to my ears, because: (i) the sound is more coherent; and (ii) they do not have the 880's dreaded small peak at 4khz (which is more problematic than peaks > ~5khz, because it is actually within the fundamental frequency range of several natural instruments).
Your amp may or may not be a factor. I haven't heard the D7. As a portable, it may or may not have the voltage swing necessary to drive high impedance headphones without shortcomings. Per my recollection, this was one of the reasons the O2 was designed to run off 2 9-volts, rather than the smaller rechargeable batteries commonly employed in a number of other portables.
Then again, I don't really buy into the whole the "HD650's are super picky" mythology. I've enjoyed mine with many different amps (albeit, all desktops). Some certainly are better matches than others though. In any event, what you are describing does seem consistent with how these headphones measure. Hope this helps some!