default torrent client _IS_ transmission, they just dont call it with its name and slightly modified it so it works with their web interface
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if you want to use a standard transmission remote gui client you just need to modify /mnt/HD/HD_a2/.systemfile/P2P/.settings/settings.json
search for "rpc_whitelist" and add your pc ip or your lan ip-range (for eg: "rpc_whitelist": "127.0.0.1,192.168.0.*", for the whole 192.168.0.0-255 range) and you'll be able to access transmission at 9091 port (standard transmission web interface dont work, some file is missing).
mind that they modified so it will ignore any user/password settings to be able to work with their web interface so it coul become a slight security weakness in your lan.