The Mystery of Speed

[#1]

My current download speed is about 1.6Mbps but I think it should be better. I base my on assertion on the following data:
My distance from my local telephone exchange is 2.27Km (straight line), 3.22Km (by road) or  4.1Km per (http://www.kitz.co.uk/adsl/max_speed_calc.php - with a downstream attenuation = 56dB).
Kitz maximum speed calculator (see attached) indicates that I should sync at about 3424Kbps giving a BT profile of 3000.
I am currently syncing at 2016Kbps with a profile of 1750.
I understood that ADSL used the bandwith from 28KHz to 1.1MHz.
Information gleaned using the DMT tool (see attached) shows that nothing above about 621KHz is being used. As I seem to be using only half the bandwidth, does this mean I'm on an upto 4Mbps service?
Your thoughts and comments would be much appreciated.

[#2]

Quote from: MrT

Kitz maximum speed calculator (see attached) indicates that I should sync at about 3424Kbps giving a BT profile of 3000.

It's a maximum speed calculator, it tells you the best you could possibly sync at with a perfect line, perfect equipment and perfect internal wiring. 

[#3]

I realise that Kitz gives a maximum speed. The maximum download speed with a 3000 profile might be about 2.7Mbps. I was just wondering what's happened to the 1.1Mbps that I seem to be missing. I'm dowloading at about 60% of maximum. Not very good!

[#4]

For tones to be used they have to be able to carry at least 2 bits of the signal (on ADSL1). Since your line can't manage that at frequencies just over 621kHz those frequencies are not used. It doesn't mean your 'up to 8M' service has been capped in any way.

[#5]

I would have said you are getting a pretty good result for 56 dB of attenuation.  But if you want to try to improve on this, the first thing to do is to try connecting your router to the test socket behind the faceplate of the master socket and see if that makes any difference.

[#6]

@spraxyt
Thanks for that piece of useful information. 
I'm surprised that nothing at all appears above 621KHz. Looks like my line could do with some equalisation or something! I wonder whether there might be a faulty filter somewhere.
@ReedRichards
If only I had a test socket. 
I've tried both master and extension sockets with and without other equipment plugged in - the results don't vary.