Choice of Phono Cartridges

hi,

1st: your existing cartridge, with elliptical stylus, has some nice upgrade options. you can change your elliptical stylus to either a VMN60SLC (special line contact); or a VMN50SH (shibata,). 

click on the chart on this page about your cartridge for options

https://www.amazon.com/Audio-Technica-VM520EB-Elliptical-Turntable-Cartridge/dp/B01M27L6P5/ref=pd_lpo_23_t_0/134-4029388-9775825?_encoding=UTF8&pd_rd_i=B01M27L6P5&pd_rd_r=09484c80-213a-49a6-a390-bb0d442c0226&pd_rd_w=gkYeh&pd_rd_wg=FNlgt&pf_rd_p=7b36d496-f366-4631-94d3-61b87b52511b&pf_rd_r=A3K65EGGFK9ZM6WHNHNS&psc=1&refRID=A3K65EGGFK9ZM6WHNHNS 

Do you know about advanced stylus tip shapes? 

life of stylus: current elliptical estimated to last 300 hours. both the SLC and SH tip shapes make much more contact with the groove (a good thing indeed), and they are estimated to last 800 hrs (2.67 times longer life). So, you can pay more for an advanced stylus shape, it lasts much longer, it really does not cost more,if you do the math.

Either of those advanced stylus (I would pick the least costly of the two) will definitely improve your audible results, and less wear on your LP's grooves, win/win.
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to clarify, you posted the number of the replaceable stylus for your cartridge. AT adds an N (needle) in the cartridge number, so the cartridge is VM520EB, elliptical stylus is VMN520EB

the EB stands for Elliptical (stylus tip shape), and B for Bonded (glued).
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How are your cartridge alignment skills? No matter what you buy, if you don't have a few inexpensive tools and acquired alignment skills, you will need to have a friend do it, or carry your TT into a shop (mask on), have them do it. You are better off practicing with your elliptical stylus, get the tools and skills for life, then you get the best sound and avoid groove damage free for life
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Once you know the cost of an upgraded stylus for your existing cartridge, then check the features of new considerations.

a lot of cartridges in that price range have elliptical stylus, which is a lateral move regarding the tip. I would strongly advise moving up to advanced shape: LC line contact; ML microline; MR microridge; SAS; Shibata, all of which are designed for MORE surface contact (edge of stylus) against the edge of the groove.
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I just put myself thru torture researching a Moving Coil cartridge (MC). Today I ordered an Audio Technica AT-33PTG/2. It has a Line Contact stylus tip on a boron shaft.

https://www.amazon.com/gp/product/B00475S0BU/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

Having decided, I feel like I got out of jail.
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Do you know about MC? They have delicate weak signals that need to be pre-boosted up to the strength of Moving Magnet (MM) cartridges, like your existing MM cartridge.
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Your phono box is very nice and will handle either MM as now, or a new MC if you do that.

best of luck choosing
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my 2 recent torturous threads

https://forum.audiogon.com/discussions/mc-cartridge-elliptical-around-0-5mv-track-1-25-1-75-affordable?utm_campaign=website&utm_source=sendgrid&utm_medium=email

https://forum.audiogon.com/discussions/audio-technica-at33ptg-ii-mc-cartridge-anyone-have-it-or-heard-it

Elliott

I have used the AT OC9III(discontinued, but still available) 
Not having compared specs in detail, it looks like the XML is similar, and with a threaded body, which the older versions lack.
Great cart for the money. There are plenty threads with satisfied users.

The Soundsmith Otello has a nicer mid/high presentation, but soft in the bass. Still a very nice cart in its price point. The SS web page points this deficit out. I use it as an emergency back up cart.

This naturally, is a subjective opinion-YMMV.

With that many albums, a more serious table setup and eventually, phonostage may be  something to consider.

Given the quality of your amps/speakers and size of record collection, I personally think you would get more enjoyment from your records by upgrading the whole turntable.

There are some great deals out there at the moment - for example Music Direct are specialling out the Rega RP8 for $1995 ( outgoing model, $1000 discount ) - brand new and ready to go. You can get a better front end second hand but why not avoid the hassle. Buy a Rega Exact and it is plug and play.

If you decide to upgrade the cartridge for now I would stick to MM/MI because they are less demanding of the tonearm ( which on the AT is not a world beater ). My recommendations in your price range would be

Soundsmith ( these are excellent at all price levels - very smooth, detailed and musical ).
Nagaoka 

I prefer these to both the Ortofon MM's and Sumiko's.
Whilst the AT MC's do perform well for the money, you wont get the best results with the AT arm.

+1 on dover's comment above

tt upgrade is best bet

try a well tempered labs or an oracle

Choosing a cartridge you must take in count an effective mass of your tonearm. Some cartridges will not work properly on your current tonearm because of the tonearm mass and cartridge compliance miss match.

Even if you can find inexpensive MC cartridge within your price range this is not final. You will have more problems with low output MC in a mediocre system. Another problem is a cost of factory re-tip (and you will have to ship your cartridge away for a long time).

Of course you can upgrade your turntable, but for every turntable you need a cartridge. You can easily start with a cartridge and if it’s a mid compliance cartridge then it is more or less universal for modern tonearms (they are mid mass).

In your situation I think MM or MI is a better choice within your price range. If you will add some NOS cartridges in your list you will get much more (such as exotic cantilevers for example) in the same price range. This is where some nice vintage MM are the best. I don’t know the mass of your tonearm (you have to check it) but a mid compliance MM cartridges from Japanese brand Victor in X1II series are superb and you will find a lot of great feedbacks from other users on audiogon about them!

Regarding new MM cartridge you can add Australian made Garrott Brothers p77i in the list.