Phono Stage, Cartridge, DeGritter...which one first?

A new cartridge on dirty records is not worth the investment. So you must have a record cleaning solution. Even a SpinClean or a DIY US device. Assuming that's in order, and your turntable, tonearm, and phono amp are up to the job, a cartridge will bring a significant improvement. However, a poor phono amp won't even allow your existing cartridge to shine. Neither will a poor support structure under the turntable. Or dirty power. See where were going here? It's all interdependent, and without more information on your system, you're not going to get the best value answer.

Just figure out a budget, get the best cart/table and phonostage. Until you've invested at least $5K-10K+or so, a US cleaner isn't necessary, IMO. Buy good records to start, and you can postpone a US cleaner. I've done this for years, and get away with a $75 Spin Clean.

The proof is hearing  the very same records and playing them on uber systems, and hearing them how they were meant to be-noise free, organic sounding and dynamic as no file or CD can match.

I only buy used LP's, so my saying buy "good" LPs to start is a challenge. That's another thread. 

Just FYI - my current system is a Rega P6 w/ upgraded Groovetracer platter & sub-platter & a Hana SL cart paired with an EAT Petit Glo phono stage.  It's all sounding pretty good but i've recently wondered if a new cart would take it to another level - then I thought about how clean my vinyl was...and so on.  I have clean power with Audience Adept conditioner, so I should be good there.