loud popping sound from left channel

Reverse the L/R interconnects at the amp input or at the pre output to see if the popping switches channels...........if so, the problem is from your preamp (or your interconnects).

If it is your preamp, it could be one of your tubes. Start switching the tubes from the left channel to the right channel until you find the bad tube.

I'm thinking your volume is turned up somewhat loud. If so, the alarm on the Adcom 545 is an indication that you have reached the maximum output for the amp. The popping sound is probably the woofer in that speaker bottoming out. I'm sure if you turn it down slightly, this will not happen. You may need a larger amp, if the speaker can handle the extra power, or a speaker with higher sensitivity to get more volume using the Adcom amp.

Thanks to all for your feedback.

Could also be the tube socket. Is the tube loose inside the socket? Good time to replace them.