@hasmarto
I see that your Donizetti amplifier is the Anniversary Edition, which is described as being fully balanced. So you were correct in not connecting the sub’s ground wire to a negative output terminal of the amp. However the manual for the sub indicates that its balanced input is single-channel, and when a single sub is connected to differential/balanced outputs of a stereo amp the unbalanced input of the sub should be used, with the connections to the amp and preamp being made as you have done. (Or alternatively, with the sub’s ground wire connected to a chassis screw on the amp).
By using the balanced input of the sub in that arrangement the sub would be responding to the difference between the amp’s left and right channel outputs, rather than to their sum as it should be. Which would adversely affect sonics, but I don’t think would cause any damage.
I presume, though, that the AA amp was not being powered via the 1800, and I’m guessing that the sub was not being powered via the 1800 either. While the BAT amp and the rest of the system was being powered via the 1800, and the ground of the sub was connected to the preamp. I’m not sure if the net result of such an arrangement might have been conducive to the BAT being damaged, but having complex ground paths between the components in combination with balanced power being supplied to part but not all of the system, with the system including a sub and two fully balanced amps, suggests to me that it is possible.
If I am correct in assuming, though, that neither the AA amp nor the sub was being powered by the 1800, while the preamp, the BAT amp, and the front end of the system were being powered by the 1800, I’m thinking that connecting the sub’s ground wire to a chassis screw on the AA amp might have been a safer approach than connecting it to the preamp. (As I mentioned, the sub’s manual mentions both alternatives for connecting the ground wire when connecting to a stereo amp having balanced outputs). Although there are a few amp designs for which the result might be a hum problem if the connection is made to a chassis screw.
Regards,
-- Al