[TenTec] A question about jupiter external speaker connection

[wt4t at reagan.com]

Reading in the jupiter manual, it states the external speaker jack is not grounded and the output is floating. I have seen suggestions that a audio isolation transformer should be put on line between the radio and speaker. Suggesting if you don't you can short out the audio amp in the jupiter. about as many also said they just plugged in their 1/4" phone plug and every thing worked fine. 
     I looked at the drawing of J5, the the ext speaker jack I see no chassis ground , is the jack isolated from ground?  as long as the speaker in the external,desk speaker 
is also not grounded to the speaker case. why would you need a isolation transformer?
 Maybe Iam missing something
73
dale
wt4t