Hi.
I have recently gotten VDSL through Vodafone but am having some issues getting my ASUS N66U setup properly.

When we rang up to inquire about VDSL we were told that it would work at our address no problem... I assumed this to be true as we are located about 350-400m away from the cabinet at the end of our street.
When the tech turned up to install the wiring he told me that the cabinet hadn't been upgraded to fiber and we would be connecting directly to the exchange (I assume we are going 'HOME---copper---CABINET---copper---EXCHANGE'.)
Either way, the Exchange is located roughly 800m away. I estimated about 817 meters (using Google maps drawing a line alone the road) and the tech estimated 790ish m.

Anyway, we got the cable installed and hooked up the ASUS N66U but couldn't establish a link. Luckily Vodafone had sent us out a modem (we had told them we didn't need one as we already had the N66U and didn't want to be charged).
So we unpacked the HG659 and got a link. 19450 kbit/s DOWN and 7520 kbit/s UP - Pretty close to what we were getting on ADSL2 (speed test of 18.31 DOWN). 

While the tech was doing this I was quickly looking at the N66Us settings and found where to enable VDSL so I switched that on and the technician swapped over the modem and ran another test (basically called up Chorus and asked for the connection stats).
This time we got about 15000 kbit/s DOWN and about the same UP.

I was told that the chances of getting any more DOWN were slim to none because of how far away we are from the exchange and I was asked if I wanted to go back to ADSL. (I was told that going back would require calling up Vodafone and getting another job lodged).
At this point I said I would leave it for a couple of weeks and see how the DLM (is that correct? Dynamic Line Management?) configured our connection.
The Tech said that was probably best and we could always go back to ADSL at a later date (I assume at no cost as Vodafone says if there is no speed increase the change back would be free).

We put the HG659 back on the line (3rd change) and the tech once again checked it before leaving.
Once he had gone I logged into the HG659, grabbed the settings and copied them to the N66U and then changed the DSL over to the N66U (4th change) however the N66U was still speed testing slower than that HG659.
I looked at the N66U DSL Log and saw that we were getting a line attenuation of 19dB for the down and over 100 for UP (The HG659 didn't give me any attenuation numbers at all).

I then went into the Administration > DSL Settings on the N66U and made a few changes.

First I changed the Stability Adjustment (VDSL) to 5dB and then set Rx AGC GAIN Adjustment to High Performance.

This boosted our connection speed to over 20000 kbit/s DOWN and the speed tests showed 20Mbps DOWN. UP was unaffected.
The line attenuation increased to 20dB however.
We left the setting like that for a day and everything seemed to be running ok although I did notice that if I started uploading the DL speed would drop quite a lot (down to around 4-5Mbps).
Then after about a day our speed dropped right off. I checked the DSL Log and say that the line attenuation was now reading 8dB but we were getting HUGE amounts of CRC Errors. So I changed the DSL settings back to default which dropped out speed right down again... and ever since its been dropping until It got to about 12Mbps.

At this point I plugged the HG659 back in and now the speeds are back up again however we would prefer to use the N66U if we could.
I'm hoping someone could explain to me why the two modems have such a large speed difference and how to set up the N66U to get the speed but keep the connection stable (The line attenuation is constantly changing and has been as low as 4dB but with a connection speed of about 12000 kbit/s).

Thanks.   Edit: Here is the CURRENT stats from the HG659
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