Ohhhh nooooo 
....don't start him up....he'll be telling you about all the "gizmo's" he's using.... 
Hehe, I know, I know. We've had PM discussions about his "gizmos."
Hey, whatever makes him happy.
Feeling curious today so I have a bunch of questions...
I often hear that a certain amp is "bright" or another is "laid back". Is this true or imagined? If true, how is this explained seeing that the frequency response of most amps is very flat?
Is it possible for the front end components (dac/pre/amp) to significantly increase the detail, instrument separation, and/or soundstage? How can they achieve these differences (especially the soundstage)?
Lastly, is the brightness of a speaker entirely captured in the frequency response chart, and if so what does it look like?
Yes, amps can sound bright or laid back; it all depends on the design and how well it was implemented. Now if you're comparing SS to tube, the difference is easy to hear IMO. Things get much harder when comparing well made SS amps IMO, though. YMMV.
Concerning the frequency response; it sometimes will show up as a change, but most of the time you're hearing increased distortion, which will not show up in the frequency response graph. For instance, my Emotiva and Butler amps sound more different than similar, but they measure exactly the same (well, at least through my speakers). I'd have to see distortion and FR measurements of only the amps, and I'd be willing to bet they differ, which would explain the different sound.
Usually the brightness of a speaker will be captured in a FR graph, yes. It may look like a rising line at higher frequencies, or there may be a dip in the midrange, thus causing the HF's to sound bright. Some people even prefer a slightly declining line from 20-20KHz; a flat line might sound bright in normal rooms. It all depends on the individual's tastes.
Finally, front end components can change the sound, yes. Will they "significantly increase detail" or widen the sound stage? Only you can answer that. In my experience, nothing increases the detail better than a good pair of speakers or an acoustically sound room. The rest is just minor in comparison. As far as sound stage, my preamp, DAC and now hybrid amplifier all contributed to a wider sound stage and more enveloping sound. Imaging also become more precise and encompassing. Once again, to each their own.
Start with a great pair of speakers and a sound room, then take it from there. Achieving your "perfect" system synergy will go a heckuva lot quicker.
