>> I understand how the attenuation place a cap on my download speed.
Its not that attenuation places a cap, attenuation gives an indication of the physical condition and length of your line from the exchange. The longer the line then the more attenuated the line will be ie the signal strength is reduced. Think someone shouting across a crowded room, the further you are away from that person the less chance you have of hearing what they are saying. You could say in
very simplistic terms that Attenuation is a measurement of the amount of signal deterioration.
>>> I note that line attenuation for my connection seems to vary between 33-40db.
Attenuation as reported by your router can vary slightly for a few different reasons.
1) Some routers report attenuation at a specific frequency.
2) Some routers report attenuation across what tones are in use.
3) Some routers report attenuation across all tones that are available for use.
Therefore its not unusual to see slight increases in attenuation as your sync speed increases. However Ive just noticed your line was reporting 40dBm when on the 8Mb 15dB profile (first post).
If you want a little bit more info theres some here:-
What is attenuation.
Decibels in the context of adsl