The 565 is much better, however the phono stage on the 565 is MM only, the phono stage on the 555 is switchable MM/MC. Good luck!
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Oh, but you might want to budget for more bench time; all of these preamps are getting pretty old at this point.
You might go new and buy the Emotiva preamp, it has all of the features of the Adcoms plus remote. I have not heard it but there are many people who like it, and it is not twenty plus years old.
The switches on the 555 are for using a low output MC cartridge. Using this type of cartridge on the 555mk2 and 565 would require an additional device. High output MC and MM cartridges can be used on all.
555 - NORM out is capacitance coupled, and LAB out is direct coupled.
555mk2 - MAIN out is the same as NORM, and BYPASS is direct coupled with tone controls and filter switches removed from signal path.
565 - has NORM, LAB, and BYPASS
The 555mk2 and 565 were introduced at the same time, with the 555mk2 being an improved version of the original. The major step up for the 565 was the phono stage, as it's line stage was just slightly better than the 555mk2.