Cartridge or Pre Amp

And, you have to roll tubes. It's like a rule. It's the law, if you want to be a true "audiophile". We do have a code of ethics, you know. 

Which ones? ALL of them-any you can get your hands on. It's just experimenting, right? Anything cheap, even used, you should try. Then, if and when you want to spend real money on tubes, you might have an idea what to get. 

I may also experiment with upgrading the stylus.

Replacement styli for AT-ML series are impossible to find nowadays, they are expensive. New styli for different AT models are not compatible and not as good at the ATN170 or ATN180 styli you may want to upgrade for. I think you have very little chance to find a better AT stylus than your beryllium/linecontact ATN150, this is how it looks under my macro lens. Beryllium cantilever with pressure fitted nude LineContact diamond. Beryllium cantilevers are no longer available in the industry. If you can’t hear the potential of this AT cartridge, then something wrong with your phono stage, maybe you need a top quality NOS Telefunken military tubes to improve the top end and clarity. I;ve bought all the best NOS tubes to experiment with my amps, but i ended up with First Watt gear and i don’t want to get back to tubes anymore. With MC cartridge like the ART-9 you will have even more problems with phono stage. Like @jollytinker i am a user of ZYX CPP-1 headamp and various SUTs along with different MM/MC phono stages.

If you can’t get the righ sound from such a nice AT cartridge then it’s too early to think about LOMC. The ART-9 is reasonably priced, but the phono stage or sut will cost you twice as much to the cost of the cartridge. When the MM cartridge sounding "dark" i would recomment to change the loading from 47k Ohm to 100k Ohm.

I wish i could find more styli myself as i am using AT-ML180 now (it’s a killer MM cartridge, one of the best ever). If you want to experiement with stylus then you can use that plastic stylus holder frame to send to SoundSmith for Boron Cantilever, i mean this one. If you want to look at comparison chart of the different AT-ML models you will find it here.

LOMC is not panacea in your situation, it will only cause more problems.
Life is easier with MM carts, some of them are exceptionally good compared to expensive LOMC.
And this is why i recommend JLTi MM/MC phono stage with its" diamond transistors", this is how it looks inside: https://scontent-arn2-1.xx.fbcdn.net/v/t1.0-9/18275023_1683323108352516_8614811338337251752_n.jpg?_n... 

Just a though...perhaps a bit off topic, cause I don't know if I would suggest a different cart, at least maybe not yet.

But what about high output MC? 

MC carts take capacitance out of the picture, which means more cable options. 

Generally speaking, where low output MC's put more demands on phono stages that MM's, high output MC's put less demands than both. 

Just, you know, something to consider. 

The 3 Laws of tubes

1. Thou shalt roll tubes.
2. Thou shalt use Herbies Tube Dampers.
3. Thou shalt use Graphene contact enhancer on the tube pins.

4th Law optional: Thou shalt cryo your tubes.

@geoffkait 

You forgot the law about recycling tube vacuum.

No mention about cleaning the vacuum.

My personal taste leans toward the use of natural vacuum instead of synthetic vacuum.  (I own a tanker car filled with liquified vacuum on a rail siding just outside Biose, ID.)

Cryogenic treatment of vacuum is essential.

Vacuum must be matched.  After all, mismatching tube vacuum just....   (wait for it....)  sucks.

My suggestion for creating natural vacuum is simple.  Just get your hands on a source of pure Hydrogen and remove the protons and electrons.  Neutrons will settle out at the bottom. 

It's possible to create even more vacuum by increasing the size of the vacuum storage vessel containing the vacuum.  The benefit of this is the increased purity of vacuum (see cleaning vacuum) and overcome the need to buy a vacuum cleaner.