SNR: Downstream 15dB+ happily reduced

[#1]

Any thoughts folks? I've got an old style master socket/faceplate without a test socket. It's got the yellow capacitor/whatever it is in there so I know that's the master socket. Line attenuation downstream is 31dB.
There's some noise on the line I can hear during telephone calls / the BT quiet line test but I keep forgetting to get a wired telephone to test with (I have cordless).
I've removed the bell wire from all of the 5 sockets in the house.
I have XF-e1 filters attached to all devices - I have 2 sky boxes, both double-filtered.
The upstream SNR on the line is 7dB.
However (a while ago now I must admit) I have plugged the router straight into the master socket and the SNR didn't drop at all - maybe 0.5dB. So I think that kind of rules out my internal wiring doesn't it?
Does the assembled wisdom think that getting an NTE-5 faceplate would reduce the SNR somewhat? I'm not sure.
Could I get my line re-trained and would that reduce the SNR?
I've spoken to Sky support (I'm on Sky <insert sky line rental package name here>) and they said nothing was wrong with the line fault-wise and I could get a BT engineer around at £100 + £6x per hour to put an NTE-5 in...
PN have said that the fault level on my line is 6mb and it never drops anywhere near that low... I just want as much juice as possible out of the line.
According to http://www.kitz.co.uk/adsl/max_speed_calc.php I should be expecting 14.5mb... which (As I've sync'd at 11mb) I could hit if I get the SNR down.
Line distance according to that calculator is about 2.2km. Samknows reckons 864 meters as the crow flies.
Cheers all.
Edit: Added downstream line attenuation value.
Edit: Tweaked title.

[#2]

Talking to myself again!
Well it looks as though that 15dB target SNR was set as a result of my unfiltered Sky box (best guess here) - as I've managed to force the router to just about 6dB. It's getting a bit close to 5 now so I've possibly been over zealous.
It was running for 15 hrs through last night and today, and I rebooted it to pick up the new 12000 PlusNet profile (Thanks Matt!). When I reconnected though - it'd sync'd at 16895700! So I should hit 14000 provided the line can hold it.
Which, fingers crossed.. looks like it might.
If only I could get some SNMP monitoring on this. Not having much luck so far.
DSP Firmware Version E.25.41.55 A  
Connected true
Operational Mode G.Dmt.BisPlus
Annex Type AnnexA
Upstream 1024000
Downstream 16895700
Elapsed Time 0 day 2 hr 8 min 3 sec
SNR Margin(Upstream) 7.0 dB
SNR Margin(Downstream) 6.00 dB
Line Attenuation(Upstream) 14.0 dB  
Line Attenuation(Downstream) 31.5 dB  
CRC Errors(Upstream) 0  
CRC Errors(Downstream) 0  
Latency(Upstream) Fast  
Latency(Downstream) Fast  
Edit:
Today 16:14 9103 kbps (1.14MB/s) 868 kbps (109kB/s)
Happy days!

[#3]

Yours wouldn't be the first problem caused by a Sky box

[#4]

Progress too far:
I was well too keen. Around 9PM last night the downstream SNR had hit 2.5dB. Needless to say that wasn't maintaining much of a connection =P It was holding, but no data going anywhere.
Set it to 10dB now and I've managed to get SNMP working with RouterStats Lite (nice piece of kit that). overnight it's been fluctuating by 1dB or so. I'll let it run all through today and this evening and see what my spread is. Hopefully that information should let me get it as close to bottoming out on 6dB as I can.
That said - I did have 2 disconnections at 6:30 and 7:00 this morning - straight from 10dB to 0dB and back up... intriguing!

[#5]

Which router/firmware are you using to be able to reduce the noise margin by that much - with mine I can only reduce it by 6dB

[#6]

Morning Jim - Billion BiPac 7402nx (http://www.billion.uk.com/product/3g/7402nx.htm) - latest firmware (553.s6.b1).
I have to say - I've had plenty of DSL routers and the like - Netgears, Linksys, etc but this one blows them all away. The functionality in there is immense.
Sadly the web gui only accounts for about 10% of what you can do with it - you need telnet and the CLI for the other 90%. To compound that further, there's no CLI manual. =P Not for the faint hearted.

[#7]

Ah - not for me then. 
I can use telnet and CLI but only with a manual  
Given all that the lowest stable noise margin on my line is 9dB so reducing by 6dB only isn't too much of a restriction

[#8]

How would you determine a stable SNR, Jim? if the SNR didn't fluctuate a great deal, and the line held for X amount of time? I suppose you have to monitor over a period.. and look for trends. Difficult (for me anyway) to ascertain the cause of a disconnect...
Suppose I should put it back to default target SNR - run it for a week and monitor.
Drop it by 3 - run it for a week and monitor.
Rinse and repeat...
Back to the Billion - It's not massively complex but its more of a 'what's this value do'.
I've got a DataBoost setting which can be enabled/disabled. A DetectNoise setting which can be enabled/disabled... Lord knows what they do but they sound interesting! Without any manual though I can't tell if they're designed for a certain type of line or service or whether they'll do more harm than good.
3 settings to Force SNR down:
AnnexAForceSNRMarginDn
BisAForceSNRMarginDn
BisMForceSNRMarginDn
Finally, they've a reputation for holding a good sync on long lines - lot of Billion users over in Aus.

[#9]

Basically by trial and error.
At present the default noise margin is 12dB and with 9dB I have a stable sync speed of 3776kbps (the maximum stable speed is between 3424 and 3850 kbps - any more than that and the errors leap up and it often loses sync over night. Fortunately that gives me an IP profile of 3000kbps.
My main problem is that there is an intermittent/occasional external interference problem which can reduce my upload speed down to about 232kbps (I think it is due to water in a connection somewhere).
First picture is current - second is when I had the problem

[#10]

Hmm - I should keep a workbook somewhere with line stats, uptimes and error counts along with the routerstats charts.
Should be able to do some analysis of that lot to find the sweet spot then.
I'm hoping I can see an Openreach van outside / nearby at some point and see if I can find a likely engineer to take some cash in hand and sort me an NTE-5 
That's a fairly significant change in upstream...

[#11]

Do you still have the telephone noise problem?
You can still raise that with BT etc (you can always say that you have a wired phone!)
They can do remote tests to check...