Regarding Dentdog’s Zesto Leto, there doesn’t seem to be any indication at the website or in the manual as to how the gain change is implemented. (Although the manual states in bold-face type, in the paragraph dealing with the internal gain switch, that "due to high internal voltages there are no user adjustable switches, only authorized representatives can make these adjustments," and also states that the "warranty is void if the product has not been used in accordance with the instructions found in this owner’s manual.")
A schematic for the McIntosh MC60 can be found on the web and shows that its gain control is simply a resistive pot (potentiometer) used as a voltage divider and connected to the input through a capacitor.
As I see it the bottom line to Dentdog’s question, and to such questions generally, is that whatever sonic effects might result from changing how the overall gain is apportioned between components figures to be highly dependent on the specific designs that are involved. So in most cases the only way to find out is to try the various alternatives in the particular system, IMO.
Does trimming the gain on the amps give me less headroom when listening at higher volumes?
No, assuming that you don’t turn the gain controls on the amps so far down that you can’t drive them to full power (when and if that is required by the music and your volume preferences).
Regarding the ZBOX, it sounds like it uses the tube in a cathode follower configuration, and has an Alps pot at its input for gain control. It is described as using "large hi-end coupling caps," so perhaps the specified output impedance of "less than 1000 ohms" is conservative, even at 20 Hz. In any event, I would think that the key to its sonic effects, in addition to possibly allowing the gains of other components in the system to be optimized, is the mention that it "works by creating a starved voltage condition in the filament of the tube that in turn causes an increase in favorable harmonics and a decrease in objectionable distortion." Although I suppose the reduction in "objectionable distortion" may not really be a reduction, but rather a masking by the "favorable harmonics" that are introduced.
John ("Ivan"), thanks for the nice words.
Regards,
-- Al