Different Bass EQ for A/V ?

I can't explain why what you observed occurs, but can confirm that this difference in bass level IS happening in my system as well ... I have asked MANY audio friends about these pronounced format differences --no one seems to be able to explain why.

Anyone out there who can shed some light on this topic?

It's the boom and sizzle that exists on sound tracks in todays movies. Remember, just about every sound you hear in a movie soundtrack is canned,(FAKE) which can certainly be unlike the natural sound of acoustic instruments recorded honestly or purely in a studio or on stage. It could also be the phenomenon known as psycho-acoustics. Perhaps after listening to and adjusting your ears to what you think sounds perfectly natural for a movie (see Mr. Lucas of THX fame)you begin to listen to music and realize without knowing or understanding why that your system is juiced up too much at a certain frequency. Your dilemma is certainly not uncommon for those audio enthusiasts who enjoy a good movie on their surround systems. However, you are learning first hand the reason for owning separate systems. The drag of having to re-adjust everytime you switch between music and movies will ultimately force you to make a decision about what is most important for you when you decide to spend your free time enjoying film and/ or music. Perhaps a very good quality equalizer from a company like TACT with preset equalizations could be the answer. If you can swing having two systems in your home, that would be the best way to go.
Good luck and best wishes!

Guys, thank you for your thoughts. Now, at least, I know that this appears to be a relatively common phenomenon. And Patmatt, my audio-only systems are separate from my HT system and this seems to be yet another good reason to keep it that way. :)

I'm still curious to know the specifics of the apparent differences between HT and audio-only bass equalization. If the audio curve is flat, then the HT bass curve must be down from flat below 50 Hz, which would require additional bass boost to achieve flat response. If no one knows the answer here, I may write to Dolby Labs and see if they can give me an explanation.