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The rectifier tubes range in price between $20.00 and $40.00 ea.  The 5U4 and 5AR4 have less of a voltage drop whereas the 5Y3GT has a bit more voltage drop.  The voltage drop changes the operating points throughout the amplifier including how the amp is biased, so you hear a subtle change in warmth from one rectifier tube to another.   There is no substitution at the time of purchase, the amp will be sent out with the 5U4 to establish a point of reference.  From there you can purchase different rectifiers if you like and see what they do to the sound of the amp.

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The cost of the VR tubes is about the same as the rectifiers.  Again, the amp will be shipped with the OA3 and OC2 to establish a point of reference.  From there you can purchase alternate tubes and see what they do.  To pick between them is purely subjective.  The differences between these tubes is voltage related, so similar to the rectifier tubes, there is a voltage drop.  The OA3 has a least voltage drop while the OD3 has the most.  To experiment with these you could buy an OD3 and compare it with the OA3 the amp comes with.  See which one you like better, and if you wish the change would have landed somewhere between the two, then get an OB3 or OC3 and see how that sounds.  As a general rule the OA3 will have the best performance, while the alternates will mellow the amp and warm the bass.

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The stepped attenuator is a dual ganged attenuator serving both channels. We charge $100 labor to install the control. This requires us to modify the chassis for the control, and shorten the shaft of the control so it will work with the TORII MKIII knobs which also have to be modified to work on the shaft.  Our cost on the control is $149 plus shipping.

Steve