While this info is specific to the connection between my NVX and my M6500, it may well apply to other TVIX and perhaps other NAS .
Normal connection (for best write to NAS) would appear to be:
1. Connect router to a gigabit switch
2. Connect NAS to gigabit switch
3. Connect LAN cable (at least cat5) from TVIX to gigabit switch
4. Connect router to phone line (obviously)
5. Connect computer to gigabit switch
Now all should be rosy

Transfer speed from your computer to the NAS should be pretty fast and for myself 35-40mb/sec
However, when accessing the NAS from the TVIX, real issues, transfer of even dvd files not sufficient for watching, jagged, frame by frame, no sound.
Now I had read a HiJack comment in another thread referring to the M6500 (which is a 10/100 device despite that 1000mb/sec option in set-up) and the problems you can face with auto-negotiation with gigabit switches. As it transpires, I reconnected through my old 10/100 Billion Router and all worked fine. So I tried a different gigabit switch (originally a 5-port D-Link and then an 8-port Netgear); and still the same problem.
The trouble is, it's great to have a NAS that works seamlessly for streaming, but, with all that potential speed un-useable for file transfer.
The solutions I have considered are
1. Keep trying different gigabit switches till I find one that auto-negotiates properly with the TVIX (retailers don't like you doing that a lot)
2. Get a managed gigabit switch (way too expensive)
3. Get a gigabit Router (that may work, not sure whether it would still be only auto-negotiating)
4. Get another Media Player (took my wife long enough to work out the M6500 - really not an option)
5. Put up with the whole thing

If anyone knows of a gigabit switch that auto-negotiates with M6500 that would be good