I've had a pair of HD6s for 1+ months and played a lot of music through them. I'm very happy with them. I certainly haven't had a problem with tubby sound. Our Audioengine A5s reminded me of Henry Kloss designs: not the ultimate but always a pleasure to listen to. The HD6s have the same virtue but they sound considerably higher up the audio quality ladder.
I tried them in living room and library room situations (20' by 15' by 9' for the library) where I'm listening in the far field where they sounded quite good. Then I moved them to my home office where I listen in the near field (2.5 - 3 feet) in an irregularly shaped smallish room. They are angled with respect to the wall 7" behind them and are about 5" off the desk. I have the speakers tilted up slightly so that the tweeters are pointed at my ears. I'll probably raise them a couple of inches and get rid of the tilt.
I listened to the Yello Devine video through the HD6s and heard nothing that I'd identify as tubby sound. Tried again with higher volume and still no tubbiness.
The HD6s list for $ 750 and sell for that normally. Audioengine had them on sale for $ 600 in the Black Friday / Cyber Monday period.