personally I'd go for the mag cart rather than a new nail on the Astatic, especially if you aren't playing 78's. I have an AT6 with a Stanton DJ cart that I put in my console. It runs one of those much-hyped Realistic phono preamps that I got for under 20 bucks. Works wonderfully. I got the Stanton cart when they were on clearance from everywhere for under 30 bucks. If I had it to do again I'd be fine with the AT3600l though. The AT6 will run happily at 3 grams.
One small mention, there are 2 slightly different headshells for the AT6. The OP's has the "long skirt" version. With some cartridges you may need to add a spacer to get the nail below the level of the headshell.
The version more typically seen with mag carts has a much shorter skirt.
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also depending on the existing mount arrangement, you might need the adapter that goes inside the headshell to convert it from single center screw mounting to 1/2" mounting. There is a short and a tall version of this too
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basically if you have the long skirt headshell and the short adapter, its probably going to need a spacer. Not a big deal, just something to be aware of. I don't recall if these are British screw thread or metric. Probably British 6BA.