The DE stock air box 06 up with a proper panel filter is the best to retain power and temp intake.
You lose power and gain noise with aftermarket open cone type
I use the Channnel Plus XL by Horstmann.
It is now over 10 years old but has proved to be a great piece of kit.
I have my central heating, underfloor heating and boiler all on different programs.
I would let the fault develop until the slip is more regular, however at 70k you have done well.
So time to start saving for a new clutch kit and flywheel.