Thought this was too easy, I may have to come up with some more mods for this one as the temp on the pi’s cpu is quite high even in idle. 
Not as much empty space as the bookshelfs.
Well having researched a bit the new pi3 does seem to run hottest than previous ones. It’s not in danger territory so will just keep an eye on it.
Did some EQ this morning and got it sounding a little more balanced.
The load on the Pi tends to be pretty low when just streaming, so perhaps it’s more the amplifier and the box that doesn’t breathe? Also, if it’s just running warm, that’s really no big issue.
One thing you could try (though it would mean taking the thing apart) is to put a little heatsink on the Pi’s CPU. But I doubt that it’ll make a big difference, the Pi 3 B+ actually dissipates heat much better than older models because of the metal top on the CPU. Which is another reason I think it’s not the CPU that is producing the heat.
I don’t think there is much room for a heatsink in between it and the amp board. It was running warm outside to when I tested it now I come to think of it. Although I did not check the temp but it as a lot hotter to touch than the previous model. I did find some others online that found their units to run at a similar temp as mine so I am not concerned. It’s not burning hot, and the location its in won’t help so will just see how it gets on. It’s seemed to have stabilized at a steady temp if a little higher than my other models.
Checked my other pi3 which is open to the elements and only has a LCD attached to it and it’s not much cooler than the one in the speaker so I will sleep easier knowing the pi3 just runs hotter in general.