When do you need a power conditioner

IMHO Never hurts to have one in any system.

I bought a PS Audio Premiere Power Plant, used, about a grand. The improvement it made in my stereo was unexpected, and I have dedicated lines. I bought another for my theater and again, I got improved picture and better separation from my surrounds. So, I really believe in these things. I even had to turn down the setting on my Darbee, a terrific product in its own right. You have a lot of things to power, and the PPP can handle ten at most. I was using Pure devices but they were really killing the sound of everything plugged into them. Whatever you decide, compare the strip to the wall. If the strip isn't better, don't use it!

I'm just breaking in a Blue Circle 2X PLC Thingee as I write this and all I can say is yes indeed, a good power conditioner makes a great difference.

All the best,
Nonoise

Yes, the right PC absolutely helps. But first of all, tell us if you have a dedicated line to your rig? I would start with that.
Not familiar with your Monster stage 2, but sometimes you need 2 PCs in a large system.