FTP speed problem - streaming from STB HDD via router over cable

I have a newly softmodded box using Krayzie 1.1 installer; XBMC 16/07/06 installed. When streaming over LAN through wireless, speeds are fine.. problem occurs when streaming from a Topfield STB HDD, (connected via USB to ASUS Router running an ftp server) accessed from the xbox by it's ftp address & cabled with cat5 (ie. no wireless between xbox & ASUS)...

When using SmartFTP from the PC, both sources will sustain transfer speeds of ~500 Kbps quite OK; the problem only shows up using the xbox to access the STB directly.. the maximum I can get it to go is 256 Kbps which causes video stutter

Having looked at the files on the E/drive of the xbox, the only config for the ftp I can find is a FileZilla.xml file -

1) Are there any other tests I can use to pinpoint the source of the problem;
2) Are there any settings I can tweak, to increase the speed?

I'm pretty sure this isn't covered anywhere (fingers crossed behind back), I've read heaps & not come across anthing like it...

Cheers,
Brian H