Installed new tubes. Now a loud pop!

Put the old tube set in and sub in one of the NOS tubes. If no popping, remove that NOS tube and set it aside, it is not the issue. Now sub in the next NOS tube, and repeat, checking one at a time, until you identify the problem tube. Then replace it.

Both channels ?

One channel. I used "new old stock" Telefunken tubes. I pulled them out to find a scorched one. Problem solved. Old tubes back in. I'll look closer next time. Now a note to seller.

Thanks