Is there anybody who knows the effects of this transformer volume control preamp to low wattage single ended triode amp such as 6bm8?
I'm not sure what you are expecting but a TVC attenuates the input signal by stepping it down, the amount of attenuation is selected by selecting different taps in the secondary of the transformer. The truth of the matter is that with TVCs, the output impedance changes as you change taps as opposed to the manner employed by all resistive potentiometers that creates a voltage divider between the input and the output and presents a constant impedance across the entire adjustment range.
What I understand is that, it increases the db of the input and output.
It does not.
In fact, it does quite the opposite. Being a step-down transformer, the attenuation is accomplished by varying the ratio of the primary versus the secondary windings, however, this is not practical so to be able to choose different attenuation levels, one must use a "stepped" rotary switch and choose the appropriate tap that corresponds to the desired signal attenuation level.
So it may increase attack to my 100db speakers. It's what I need for now to improve performance of my 6bm8 SET.
I doubt it. No type of volume control nor preamp can do that to an amp, especially to a tube amp. Most tube amps are "lazy" risers, for they tend to compress the signal and prevent distortion. This is, for what it's worth, is sometimes what is sought after from them. But for your predicament, perhaps a further study of the input section of your amp and stiffening your psu can improve the "attack" you are referring to but at the expense of compression and distortion.
You change one thing for the sake of another thing applies here.
Is my speculation correct? Kindly enlightened me.
I hope I didn't lost you up there, it can be quite technical and I was trying to be as objective as I can in my response rather than be subjective.
If there anybody who can locally manufacture this TVC? How much? Thank you. 
I am not aware of such but, IMHO, you may want to reconsider, the reason these TVCs, and moving coil SUTs are priced like that is because of the materials required and the strict tolerance that they must go through during manufacturing. You get what you pay for in this department.
Anyway, you may want to take that last paragraph with a grain of salt.
