I'd second Arni's suggestion about using a configuration that leaves your existing speaker connections to the Celestions as is and running a second set of speaker cables from your amp to the high-level inputs of a powered sub. The second set of cables, the ones to the sub, don't need to be of the same gauge as your main cables since the powered sub is drawing very little current over them, meaning you don't have the expense of two sets of high-quality cables as you would if you go first to the sub and then on to your mains.
If you have only one set of speaker terminals on your amp but have five-way binding posts you can use spade connectors for to cables to your Celestions and banana plugs for the cables going to the sub.
I've used that setup with a pair of ProAc monitors and an HSU VTF-2 sub and found it provided easier sonic integration than using a high-level connection from pre-outs on the amp.
If you have only one set of speaker terminals on your amp but have five-way binding posts you can use spade connectors for to cables to your Celestions and banana plugs for the cables going to the sub.
I've used that setup with a pair of ProAc monitors and an HSU VTF-2 sub and found it provided easier sonic integration than using a high-level connection from pre-outs on the amp.