The hum sound could also be caused by the AC current used for the tube heaters. Some owners have eliminated the hum by hooking up tube heater pins 4 and 5 and the step down resistor directly to the power transformer output, so it is off the main circuit board. Several websites have details on how to do this.
Output Impedance - There is also a signal impedance issue with Tube Drivers and Real Tube pedals (not so with the USA made Tube King). As a general rule for a pedal board signal chain, you want a low impedance output connected to a high impedance input on each pedal to allow the widest spectrum of audio bandwidth to pass through the chain in order to keep your signal from degrading. The Tube Driver has a buffered, high impedance input, but no output buffer. The passive tone circuit hangs a high impedance signal on the output, so changes in loading and capacitance from long cables or other pedals in the signal chain can result in a loss of bandwidth, altering the tone in a negative way. High volume settings can place a load on the Hi and Lo controls, so different volume levels can affect the output sound differently. With improper impedance, some users find that turning the treble up can result in a loss of bass. Not everyone encounters this, and it will vary depending on the Tube Driver settings, tube used, and the load from the other pedals your signal chain.
You can check if there is a tone difference by comparing the sound when playing the TD in your pedal board (with all other effects switched off) to playing with just the guitar into the TD, directly to the amp. You can also hear this by taking two identical Tube Drivers, plugged in back-to-back, directly to the amp. Turn one on and play it at high drive and volume, then swap places keeping the same one switched on, and you can hear how the high impedance output from one changes the sound when going into the other.
So how does one balance the Tube Driver impedance in the signal chain? Placing a buffered pedal or buffer/line driver with an input impedance around 1MΩ immediately after the TD, or non buffered pedal with a similar high impedance input, can help the issue, as can simply placing the Tube Driver in a buffered bypass loop that includes an output buffer. Using a lower gain tube like a 12AU7 can also help reduce the output impedance of the Tube Driver.
Faulty Footswitch - Another issue is that the foot switches BKB used in the 2007/2008, and some earlier Tube Drivers, were unreliable. The switches in my 2007 and 2008 TDs broke within a year of use and had to be replaced. I don't know if Butler has since changed to a better switch, but he was aware of the problem and was offering replacements at the time - unfortunately the replacements were the same brand of unreliable switch.