Dear Mikeba316: I agree with Lewm, the more important subject is the tonearm/cartridge combination. On which TT?, it almost does not matters at all always that the TT be a " decent " one as your MK3.
Why do you need a new cartridge when you own the Puritas?, is something wrong with or it is only that you are whealty enough to try a new expensive one even if maybe will be a different but not really better performer that what you own?
If you want to try a different cartridge sound maybe the Colibri is a good alternative before the ones you name it here and for a lot less money.
Oh yes, the Colibri is not the " new kid on the blok " but I can tell you that those new " kids " have a very high challenge to beat the Colibri and IMHO your Reed tonearm could be a very good match for it.
In the other side and if I was you my first move could be to change the phono stage for a better non-tube ( especially with LOMC cartridges. IMHO: Phono stage is the worst place for use tubes and even worst with the ones using step-up transformers. If you decide for one of this units then it does not matters which cartridge/tonearm combination you choosed because you never know how that cartridge really performs.! ) one and please don't take for sure the advise of audio items seller. Which seller can speaks bad of what he have on sale?
The Phonolinepreamp IMHO is almost important as the cartridge/tonearm match, even more important than the TT.
You have several alternatives to improve your audio system quality performance level.
Money is always important but more important is the knowledge level on what to do and why.
Your actions IMHO must be " induced " by your specific music/sound targets/priorities and not because this or that item is the latest expensive one to " impress " other people. First than all think on what you want, think how to meet/even your references.
Well, only an opinion.
regards and enjoy the music,
R.