Do preamps have a material affect on high level sources?

It alters the signal since said signal travels though many resistors and various other bits of electronic stuff. Also tubes or transistors.  Thing is, modern preamps are so clear that even several of them in series hardly is even noticeable. 
The direct line from the source may be good, or there may be a problem depending on the design of the source. Voltage and current ate two parts.. Not enough current and the sound from a source may be weak or thin. Plus the volume control. If digital, problems at low settings unless specially designed (IE it multiplies the base signal and then interpolates so not info is lost).. I personally like preamps and own five in my one system. Typically two maybe three are in use at a time. I can use just one, but where is the fun in that?

I would add to the excellent explanation above that the preamp acts an impedance buffer.

I think that the preamp does the most violence to the signal of any component, including speakers. Why? First because it manipulates the signal while it is at its lowest level, and most fragile, anywhere in the chain. And second because, unlike transducers, electronic distortions have a, for want of a better term, a synthetic signature that is at odds with the fabric of the music. YMMV.

Thing is, modern preamps are so clear that even several of them in series hardly is even noticeable.

I generally agree with Lizzy, but in this case I must disagree.  IME, the preamp has a HUGE effect on the sound you ultimately hear from your system.  A lot of what you hear will be based on if your DAC can properly drive your amp, but there is no doubt a preamp will absolutely have a huge impact on what you ultimately hear.  I hope Ralph at @atmasphere will chime in here to check me, but my experience with several systems tells me the preamp is crucial in the ultimate sound quality of a system. 

I would say sure a preamp is important. but no more important than ANY other part. Amp, speakers, source... cables, cords.. All have to fit together and all have equal importance. Any one of them can ruin the music. And none of the is more important than the other. 
To me claiming one part is 'more important'? well why not just use that alone. if it is so important, then why bother with the rest? Oh nothing works? gee.      
OK say I use all equipment that costs $5K a pop. Then swap ANY one of them with a $500 item. I can promise you, the $500 preamp is NOT going to be the worst sounding item change.

vjpacor
 It would seem that a preamp is merely attenuating a DAC output. How can it alter the signal?

The fabled perfect "active preamp" sound, has the "the sound of a piece of wire" and it does not exist.

Any active amplification circuit, whether it has gain, zero gain or negative gain. Will have, and add, it’s distortion to what the source originally has feeding into it, and they all sound different, there is no "perfect preamp"

If you feed your source direct into the preamp and it has a digital domain volume control, this is about as perfect as you can get, so long as you don’t "bit strip" with too low a volume setting on the digital volume.


Cheers George