You are right about the attenuation and approx length - at worst the line could go a little round the houses and be maybe 800m long or possibly 1km which would still give an attenuation in the low 20s.
You will not get much joy pushing on the downstream attenuation front but need to focus on the upstream which should be lower that downstream. If you can get that sorted, gut feel says that any problem on downstream will also be resolved.
Have you managed to speak with a Coach or Manager on Tech support? If not insist on it and then use that as the escalation path.
Do you know which call centre you are dealing with?
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