After two days of struggling with the issue, I, too, found the issue to be with my Onkyo receiver. Here's what I wrote this evening in the BD review:
"...just when the soundtrack should have exhibited its most-powerful impact during big action scenes, it tended to soften up and lose some of its punch, the dynamic range seeming to become constricted. Just to be sure I wasn't hearing things, I compared several passages to the regular Dolby Digital 5.1 track on the standard-def version of the movie, and, sure enough, the SD disc had the punchier sound.
After two days of fussing with the question and talking to equally perplexed fellow reviewers, I discovered that the problem was in the way my Onkyo 705 receiver was handling this particular disc's TrueHD audio output (although the receiver has never had a problem with any other TrueHD soundtracks, a multitude of them, in this regard). The TrueHD track on "Iron Man" was apparently triggering the receiver's dynamic-range compression function. There is a button on the remote for "Late Night" listening to change it back. However, not wanting to deal with the issue if it ever cropped up again, I changed my Panasonic BD50 player's output from bitstream to PCM, thereby letting the player decode the audio rather than the receiver. Now, everything sounds fine. Indeed, "Iron Man" sounds great in TrueHD, with plenty of punch that I'm finally able to hear."