1. Is there an added value in changing the Sub?
2. Is Paradigm Monitor Sub 12 an optimal choice? Or???
3. Why?
Unfortunately, there is no good answer to your questions without some other information:
How big is your room?
Is the PS-1000 able to give you the bass you want now?
Did you run ARC?
How does it sound?
Your PS1000 has one half the amp power of the Sub 12, and a 10" driver instead of a 12". It would safe to assume the Sub 12 would out-perform it, but do you need it?
Many here will probably recommend a variety of subs to replace it with. If it needs replacing, or if your room needs a 2nd sub because of size or your personal tastes, the Sub 12 is a nice unit. I have a small listening/theater room (1400 cubic feet) which a Sub 8 fills very well.
I don't know if you listen to a lot of music or just watch movies. If music is a priority, your PS1000 sounds fine in your room, and you have money burning a hole in your pocket, I would suggest upgrading the front left and right speakers. Staying in the Paradigm line, perhaps the Prestige 75F's or 15B's. With ARC and all of the speakers running the same X-PAL tweeter, sound integration should be a breeze.
I'm running an MRX520, Prestige 75F's, Monitor Center 1 v7, Surround 1 v7, Sub 8, Oppo 103D, Panamax MR4300, and a Samsung UN555KS8000 4K display. The 75F's and Sub 8 in 2 channel playback are beautiful. Having run ARC several times (at start up and again several months later-with minor differences), the Sub 8 is audibly invisible. All low end material appears to come from the towers. I actually set up an input using just the 75F's so I could switch back and forth to hear the difference with/without the sub because the sound was so seamless in 2.1 playback.
Good luck, and enjoy!