@FramJamesgot 

Unlikely its anything to with the slower DSL speeds, more likely that DSL modem / routers are not as equipped as Cable Router.

As DSL routers have a modem, where Cable Routers do not this adds additional $$ to the device.  Therefore to keep price down it may well be that the spec overall on the DSL is lower and therefore uses less power.

Generally i find Router only (Cable Routers) are much better devices than DSL routers..

That just my 2 cents :)