First off some Gemini AI derived info:

Based on your serial number K8X548, we can break down the vintage of your Accuphase M-60 quite accurately.
Using the classic Accuphase dating convention for that era:
Serial Decoding: K8X548
* K: This is the model series prefix used for the M-60 and other units from that specific mid-to-late 70s production run.
* 8: This represents the year of manufacture. For this series, it corresponds to 1978.
* X: This is the month code. In the Japanese system of that era, Accuphase often used X, Y, and Z to represent the final months of the year. X typically stands for October.
* 548: This is your specific unit's production sequence number.
Your amplifier was likely manufactured in October 1978.
Why this matters for your repair:
By 1978, the M-60 was at the peak of its production cycle (it was released in July 1975). This means your unit benefited from any "running changes" or minor circuit refinements Accuphase made during the first three years of the model's life.