Sorry for beeing away for a while again...
You are right the subsonic.sh is located under /volumeX/@appstore/Subsonic
simply edit it using vi (pre installed) or any other editor you installed (I prefer nano btw.)
Change the https port to 0 and save the edited file
Restart Subsonic
The DSM link will work because it always points at localhost:4040 which is the default http link.
I hope I could help, if you have got any further questions feel free to ask.
To the general NAS/DIY Server discussion: I think everything has been said before there are many pros and cons on each side. If you have anything new to say about this, please write it down at one of the many topics opened for this discussion. I would like to keep this one for news and questions about Subsonic running on Synology devices.
Thank You, Eric