Anyone Know Where I should Look to Figure Out This Hum?

The first thing to do is to get a ground tester and use it to check the ground and polarity of all your outlets. If they all test fine, then disconnect the main cable line that feeds the home and observe if the him goes away. This should eliminate a ground loop created by your cable/ISP. Just because your outlets test fine for ground and polarity does not mean that you are gremlin free. Any house hold appliance could be causing the issue and back feed the hum into your system. I know this because my system did the same thing and I traced it back to my new cooktop. I did all of my suggestions above, installed a ground loop isolator, bought a $2500.00 surge/line conditioner: made sure the entire system is routed through it and the hum was still there. I even had my electrician rewire the cooktop which improved but did not eliminate the issue. His conclusion was that my 65 year old electrical mains is out of date and out of code and needs to have a new ground installed. Luckily, the hum only rears its ugly head when the cooktop is on so I power the the system off when cooking. I did purchase IFI AC Line Purifier for $100.00 and so far I have not heard the hum but it is only a band aid until the electrical mains can be addressed.So the lession here is that a common appliance that you do not think is a problem is the problem, finding is the hard part.

First couple days, silent. Now this. It's coming from both sides equally, turning up the volume makes it louder.

My suggestion is unplug everything from the unit, check inside the unit for any loose connection and make sure all tubes are firmly insert to their sockets then reconnect the unit to your system. From the picture I can see some (2?) cable with screw-in connectors inside the Lamm LP2.

https://www.lammindustries.com/PICS/LP2/modified/LP2%20inside.jpg

Yeah, done all that. Swapped all the tubes now. It's definitely not a ground hum or related to a tube. Waiting to hear from Lamm.

@dhcod  That isn't a ground loop or bad ground. Those both produce a buzz, and what you have here is a hum. Its a defect (like a damaged filter capacitor or bad solderjoint in the power supply). So the preamp is going to need service.

Thanks so much for this confirmation. Now I just have to figure out how to get it repaired without shipping cross country. Can't find a schematic online so there may be no other option unless Lamm will provide one. Hopefully they'll respond soon.