To JakeZ, sorry for not replying sooner. The rectifier (tube or SS), rectifies the AC to DC voltage. After this, the DC voltage is further smoothed with R-L-C circuits, using capacitors, resistors and inductors. The purpose of the voltage regulator is to further smooth DC, but MOST importantly, it regulates the output voltage, keeping it rock-steady, no matter what happens to the incoming DC voltage.
Fluctuations in the AC mains voltage will cause the DC voltage to move up and down by some amount. In tube circuits, this results, in varying amounts of DC at the output of the tube circuit, even though it is cap coupled. The output caps do block DC, but slow moving fluctuations in DC voltage pass right through the output caps, resulting in a DC voltage that is moving up and down at the outputs. This could be 10's of mV, to 100's of mV's. This can cause problems at the input of preamps and amps, particularly if they are SS types!
When the regulator works properly, no matter what the voltage coming into the regulator does, provided it is within the headroom range of the regulator (10%-20%+ ideally), then the output voltage will not change. This allows the circuit to operate more linearly and also prevents any DC at the output of the device.
I find that this translates sonically to better bass and overall control and solidity of the sound!
I hope this helps.
Dan