Will changing gain affect frequency response?

Claims are a bit out there.
How can a tube buffer with 1000ohms output impedance, be a better impedance match than a 100ohms or even less output impedance source with it's own VC, into say the low 10kohm load  of one of the Pass Labs poweramps? 
And this 1000ohms output impedance will be even higher at lf as it's bound to be capacitor coupled!

Cheers George

The preamp has two settings for gain, set internally, of 3 db and 12 db and came to me set at 3 db.

Back to my first post, which everyone overlooked, how is this reduction achieved? Is it via the feedback network??
If so, this can influence the tonal quality, of where these gain setting positions are used, and they have nothing to do with the vc knob being high or low physical position for the same given volume.

Cheers George

NOT talking about increasing the volume at the listening position

Increasing the gain by moving the jumper to the highest position

(keeping the volume the same at the listening position)
In some cases this gain change in a preamp/amps/cdp/dacs with links can be done buy a change in the negative feedback ratio loop.

And if you change the negative feedback, you change the things like the distortion, frequency response, stability, damping factor, slew rate and many more factors.
(In most cases the less the negative feedback, the more gain you have)

So what I’m saying is the "brightness" thing your hearing may vey well be the change in one of all of these factors, if these links apply to the negative feedback.

I know in a few Cary/Wadia dac/cd players it was done this way to have the user use the digital domain volume control used only it the top quarter of full, so the risk of "bit stripping" was minimalized.
Cheers George