Most parts of WinMx use encryption but the file transfers however are the exception.
This legacy design decision allows an ISP using basic or even quite complex throttling equipment to look for a specific set of transfer messages, with these specific strings detected and on the default WinMx ports its pretty much certain when a file transfer is about to take place.
They can block file transfers on any port if they wish to but odds are they use the basic catch-many settings and let the few odds & sods port changers alone as this saves them a lot of extra work loading on the throttling equipment or software.
If your working ok on some off-the-bat ports then simply stick with them unless theres a problem again, its not common for ISPS to block stuff below 1043 so if theres a further issue drop down as low as you wish, above port 80 / 81 though of course