"Replaced input tubes in preamp and amp.
Although I did notice that the crackling did become less after changing input tubes in the amp."
With the system powered up and making the noise, very carefully move the tubes around in their sockets. Don't pull the tubes out, or even go up and down. Just move them a little bit from side to side. If the noise changes, that may indicate an issue with one of the sockets. If you have some contact cleaner, try that as well.
Although I did notice that the crackling did become less after changing input tubes in the amp."
With the system powered up and making the noise, very carefully move the tubes around in their sockets. Don't pull the tubes out, or even go up and down. Just move them a little bit from side to side. If the noise changes, that may indicate an issue with one of the sockets. If you have some contact cleaner, try that as well.