That’s a good point about the transformer. If the hum stays the same regardless of the volume knob, it’s usually something internal or just dirty power.
Before you start cracking the case open, it might be worth checking exactly how loud the floor noise actually is. I’ve used https://noisemeter.co/ on my phone before just to see if a ‘buzz’ is getting worse over time or if I’m just overthinking it lol. It’s a handy way to get a baseline reading before you move the amp to a different circuit or room to test the power.