I have had many e-mail questions sent asking why the London Power Scaling sounds so much better than an attenuator. Here is an excerpt from an e-mail reply I sent to a customer:
Finally, how does the London Power Scaling affect the tone compare to using an attenuator?
The London power scaling is far superior to any attenuator and is a much better way to reduce output level while retaining great tone. The power scaling circuit acts like a DC voltage Variac just over the B+ / screens (IE power tubes). This allows the power tubes to overdrive earlier without changing the tone of the amp. The downfall of any attenuator is that it uses a huge non reactive power resistor in parallel with the power amp/output transformer to load it down, thus lowering the volume. This in turn greatly reduces the high end frequencies, killing all the fragile harmonics and leaving you with a tone that is usually thin, choked and buzzy sounding.