The 2wire 2700 has an excellent modem at the front end - good for long and noisy lines. Most in the UK are BT badged and need an easy minor tweak to unlock them. You need to ensure you get a 2700 not the later 2701.

The 2700 only has 100Mb ports - not an issue if you are to use it as a modem only.

Happy to tweak it, currently it is +/- 5dB. Should I increment up or down and how far should I change the number?

Experiment 1db at a time, I cant remember which is +/- as the asus chipset has a funny way I think it is reversed to what you would normally think.