My guess is the better the cable, the better the balance of the twist,
so the lower the RFI. If that were the case, then maybe going cat 6
would be the way.

Also thinking out loud, it seems with DSL you should filter the analog
signals as soon as possible, i.e. where it enters the house rather than
at the phone with those in-line filters. I'm sure the DSL on the crappy
phone twisted pair radiates a bit.