Right, this is where things get tricky to explain. The big difference in tone is that the silicon chips are miles better at working as a rectifier when compared to variable and fragile tubes. A silicon chip can provide the perfect amount of power through the entire system without failure as long as it is getting the right AC current from the wall.
This means you will get the same amount of power running through the amp no matter what changes you make. This is great for stability because as long as you have decent, clean power your amp should sound the same every single night.
A tube rectifier is a lot more fragile, they break easily and wear down over time. Plus they aren’t even really that good as rectifiers. In some amps they work perfectly, but when you four 6L6 power amp tubes running extremely hot the rectifier tube will not be stable and will affect the sound of the amplifier.