Atari,I think its not a matter of what materials are used from the outside of a boombox.
What plays a big role in how good a boombox in sounding terms is whats used inside it.
The general rule says , the heavier a boombox weighs the better it is sounding.
Late 70s boomboxes until very early 80s are basically considered well build.
This is also the Golden Era of these machines, There are ofcourse exceptions
and good boomboxes where made up until the mid 80s but this is not the rule.
Well Build CD-TAPE Boomboxes were also made in the 90s and early 00s but thats a completely different era
which was not focusing on the classic looks and style was altered to match the demands of the period.
Now late 00s and 10s have seen a rise of units using bluetooth and SD cards
or other wireless protocols to transmit sound but were considerably smaller and lighter.
This made 99% of new boomboxes sound bad among other problems like mp3 digitised music is
ofcourse worser sounding than every analogue source in which you can hear every single organ part that you
want instread of a muffled monophonic sound.
Just my opinion !