From searching I’ve found this mentioned before but unfortunately there were no follow ups so forgive the repeat.

I've just picked up a pair of Ikea Symfonisk bookshelf speakers for the rear surrounds and am finding them to be a great complement to my Arc and Sub for watching TV. I have been experimenting with the Music Playback setting (for non-TV listening), and its Ambient and Full options, and having tried both I am definitely leaning towards Ambient as I think I prefer the slightly more localised sound that it allows, and in contrast the Full setting just sounds a bit too much 'everywhere' for my liking.

However, I've noticed an unexpected downside - the Ambient setting appears to reduce the sub output slightly for some reason. I first suspected it audibly but it became much more obvious when holding my hand over the sub's output hole where it is easy to actually feel the difference.

Why would it do this? The Music Playback option description implies that it only affects the surrounds so why is the sub being tweaked also? I could perhaps understand if it was the opposite behaviour e.g. if Full mode lowered the sub slightly so as not to overdo things given that a full-spectrum feed was being provided to the rear speakers in addition to the sub handling low frequencies, but this isn't the case.

Presumably this is by design, but why? (Or is it a bug?)