adsl uses various different frequencies. Downstream uses higher frequencies than upstream and the 2 are kept separate to avoid crosstalk. The type of modulation used in adsl ensures that if noise is encountered at a particular downstream frequency then it will at least attempt to transmit some data on different channels (bit swapping). However there becomes a point when some channels are way too noisy and the router has to 'shut them down'. This shows as a reduction in the overall SNRM and/or lower sync speed.
The above is a simplified explanation, but gives the gist of it.
You cant really boost your output power as such* since this is negiotated between the router and dslam, mostly controlled by the dslam. If you output power was too high then it would drown out neighbouring lines and cause cross-talk for them.
But you are kind of on the right track because With DMT youre changing your Target Noise to a accept a lower SNR figure, which will allow you to sync at a higher rate.
Some routers do allow you to over-ride the target SNR. Using the DMT tool is the easiest way of doing so.
Ive not personally heard of a specialised BT RFI choke - theres something called RF filters that BT will install on some lines if you live near a radio transmitter. But be aware RF2s have a drastic affect on adsl, and RF3's will/can lower your sync speed since adsl and AM radio use some of the same frequencies.
The other thing I think they may be talking about is the I-Plate? This stops interference on the bell wire, but you can sort that yourself for a cost of £0 and a little bit of time by
removing the ring wire yourself.
Some other users have also tried RF filters of the type you can get from Maplins, its not something Ive actually experimented with so I cant comment on that Im afraid.
