Cartridge burn-in

I've now installed about a dozen MC cartridges in my setup, from brand new. Some sound spectacular right from hour #1, and others start out decent-ish and then show improvement over the next 20 - 50 hours. I've even experienced both types from the same same brand. But I haven't ever had one sound "cringe", yet.

I don't think I've ever heard anything go from cringe to good via break-in. But to be fair, I lose patience easily and have given up on some components (not cartridges) before full burn-in. 

There is some logic in what @lewm says. Rubber has a short memory. Just over night the Porsche will develop flat spots on it's tires so for the first several miles the car rides rougher until the tires warm up and the flat spots resolve. I do not know if this is the case for the rubber used in a cartridge's suspension. The MSL Signature Platinum sounded great out of the box. If there is a change I do not hear it. Same is true on warm up. Esls for certain break in as the diaphragms loosen they become more dynamic and will produce better bass. Cartridges being electro-mechanical devices could have a break in period but I do not hear it. IMHO purely electronic gear does not break in. It is what it is right out of the box and first impressions are always the right impressions as the human mind can accommodate to almost anything.  

Regarding my own MSL Gold Signature unit, it sounded very good right out of the box but nothing really specail (as I was hoping for). After 10 hrs it started to open-up and relax but after 25+ hrs OMG it was like another animal. Now I understand what a WONDERFUL cartridge this truly is. Even after 35 hrs it is still getting better, the importer told me it will be fully broken-in after 50 hrs but even now I'm a very happy camper. 😉

Other cartridges have never taken this many hrs to come alive like the MSL, at least this is my experience. 

It has been a while since I have put a New Built Cartridge into my system and almost twice as long since I put an 'off the shelf' design New Cart' into my system.

Prior to the New Built Cart' I was using a SS Phonostage and Hybrid Valve Phonostage. Since the New Built Cart' has been added, it is working mainly with Valve Input/Output  Phonostage.

When using a New Cart' and assessing it, I recollect the Valves would 'Pop' to their prime performance quite noticeably after a period of use, and the Cart' would continue to show subtle changes towards what was to be the prime performance, from recollection it was the lower frequencies that were becoming more gathered and real in the capable of doing it right area.

I don't recollect the Cart' going backwards as the usage progressed, when the Valves 'Popped' the Cart' always seemed to be very close to the last recollected bass performance.

It was a later experience of demonstrating the Cart' in comparison to another Higher Up the Range Model from the same Brand, using the same Headshell>Tonearm>TT and HiFi System, that really showed how improved the Rebuilt Cart' had become.    

To be clear, I’m not saying that a bad sounding cartridge gets good once it’s broken in, although in individual cases that might happen since judgement is almost entirely subjective. I am only saying that cartridges that are either brand new or have been in long term storage audibly benefit from warmup. This probably has nothing to do with flat-spotting, a well known phenomenon that can occur with any car parked for more than a day or two on concrete or any very hard surface.