The problem with both of those older amplifiers is likely just a noisy 2N3440 or 2N5416 regulator on the audio circuit boards or their connections to the boards.
These are the small round button shaped transistors with the big black heat fins epoxied on. They are the top transistors in each row of three on each side of each board, one of each per channel. Often the issue is only degradation of the solder connections from these transistors to the PC board and can be fixed by reheating each of the three connections per device and adding a small bit of solder. The issue occurred because the transistor leads are very small in diameter so there is not much surface area between the leads and the solder pads to conduct heat away. Over many years of use (especially if the amp is left on all the time) the solder connection or the transistor can degrade. The result is noise at turn on, going away as the devices warm up and the now marginal solder connection warms up too and makes better contact.
The permanent cure is to replace the transistors and instead of the normal process of solder the connection and cut the excess leads off, instead bend the leads down flat on their foil traces and then solder a significant length of the transistor leads directly to the foil. This provides a much larger thermal path for the heat from the transistor to be conveyed into the foil trace and dissipated much better long term.
Of course we never had this problem until there were many units in the field working just fine for many years. Everything built in recent times has been resolved with either the fix done up front, or in the case of the big Synergy amplifiers, using different regulator techniques completely.
The cost to fix, depending upon the layout, is an hour and a half labor (more or less) and parts, typically about $150 plus return shipping.
Regards,
Frank Van Alstine
