B & K ST 140 repairs

Have them check the bias. This happened to me back in the late 1900s when I owned this amp. Bias drift was the cause.

just the fuse holder is broken? And the amp isn't blowing the fuse? you could do it yourself for about $5 in parts. Alse a shop may likely charge you their min rate for the work - usually $60-80 is my guess. The true cost of the work is $5 for parts, 10 min for labor, so maybe they cut you a break.