When do you need a power conditioner

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.

Good advice from Tls49. If you are in a house, 2 lines would be easy and recommended. BTW, "dedicated line" means your system would be separated from the rest of the electrical and have it's own breakers. No more interference from sharing AC with the lights, fridge, etc.

If not able to run a line, the right PC would improve the sonics of a system and would provide a lower noise floor. Research the archives, lots of info.