If you know what you are doing, it should be very easy to bypass the selector switch and have a single input. You should be able to do this and not do any permenent damage to the integrated. You could easily solder the selector back into the circuit later.
However, if you want to upgrade your whole system, I would highly recommend the Quad 99 CDP CD player which has an excellent remote controlled variable volume. You could use it with the power amp of your choice for an excellent system. Heck, you can even get a Quad 99 or Quad 909 power amp for a great priced used and have an excellent all Quad system for a decent price.
These components cost more than the Jolida and Cambridge, but they are excellent and could get you through many years without the need to upgrade.