You folks have been great and I do appreciate the time you've all taken. In response to one query, the MEN220 is new, and I had had three of these tube replacement occasions before I installed the unit between the C220 and the MC402.
One other thing came up this afternoon. As I'd mentioned, I had had a "trigger" cable connecting the C220 to the MC402 but not one connecting the MEN220 to the two other components. When I had previously powered up the C220, the MC402 powered up without incident. Now I have added another cable to get the MEN220 into that trigger chain (and have ascertained that the connection sequence is proper) but NOW there is an audible pop in the speakers within three seconds of powering up. There might be no connection between the two issues but I bring it up in case it provides any sort of usable evidence/information.
One other thing came up this afternoon. As I'd mentioned, I had had a "trigger" cable connecting the C220 to the MC402 but not one connecting the MEN220 to the two other components. When I had previously powered up the C220, the MC402 powered up without incident. Now I have added another cable to get the MEN220 into that trigger chain (and have ascertained that the connection sequence is proper) but NOW there is an audible pop in the speakers within three seconds of powering up. There might be no connection between the two issues but I bring it up in case it provides any sort of usable evidence/information.