This does not seem like an induced hum from your description of the sound. If you want to confirm that it's not your sub, disconnect it. If the problem remains, see below - if it's gone, come back and tell us.
If this doesn't seem to have anything to do with the sub after doing the above test, I would suggest swapping your speaker cable left side for right side at the amplifier, to see if the problem then travels to the other speaker. If not, then suspect the speaker. If it does, then do the same channel swap with the interconnects going into the amp. If the noise does not migrate again when you do this, suspect the amp. If it does, reverse the leads going from your source into the preamp. If the problem stays in the same channel after that, suspect your preamp (this is a likely culprit, since yours uses many tubes - this could easily be a tube noise). If it switches, suspect the source. In general, rice-crispies sounds are usually either from small signal tubes going bad, or bad connections at jacks, although other possibilities certainly exist. But try to rule these out first. If this test leads you to suspect a bad tube somewhere in the pre, just do progressive tube swaps channel for channel, swapping only one R/L pair of tubes at a time to limit your variables until you've identified the bad one if it exists. If any of this is helpful (or if it's not), please come back and tell us what you've done and found. Best of luck!...
If this doesn't seem to have anything to do with the sub after doing the above test, I would suggest swapping your speaker cable left side for right side at the amplifier, to see if the problem then travels to the other speaker. If not, then suspect the speaker. If it does, then do the same channel swap with the interconnects going into the amp. If the noise does not migrate again when you do this, suspect the amp. If it does, reverse the leads going from your source into the preamp. If the problem stays in the same channel after that, suspect your preamp (this is a likely culprit, since yours uses many tubes - this could easily be a tube noise). If it switches, suspect the source. In general, rice-crispies sounds are usually either from small signal tubes going bad, or bad connections at jacks, although other possibilities certainly exist. But try to rule these out first. If this test leads you to suspect a bad tube somewhere in the pre, just do progressive tube swaps channel for channel, swapping only one R/L pair of tubes at a time to limit your variables until you've identified the bad one if it exists. If any of this is helpful (or if it's not), please come back and tell us what you've done and found. Best of luck!...