Is it reasonable to conclude that either these speakers are defective, or they just need gobs of power to produce descent bass?Well, you're feeding them with 300W capability per channel, so normally that shouldn't be a problem (unless the impedance below 80Hz or so is very high which is unlikely). UNLESS, indeed, the adcom has trouble driving those 10" woofs: i.e. the sound intensity of the top part of the spkr is much higher relative to the low part, which means that you can't "hear" the bass. A solution would be to borrow another two channel amp fm s/one and try it on the woofs. This is probably the problem.
You may, if you wish, play the spkrs connecting ONLY the woofs and listen to what's happening.
You may also be experiencing a null at some upper bass frequency due to yr room. For that, there's little to be done (other than room treatment), but experiment pulling the speakers ~1/3 into the room to gauge differences, if any. Also keep them far from each other & slowly move them closer until you hear the L&R low midrange sound coupling.
And good luck!