If you do not want the preamp to add anything to the sound then consider 2 preamps that I recently bought.
The Benchmark LA4 or HPA4 ($2500 - $3000) add no sound and have some very useful features. They also work well with a variety of amps. So what you hear is the source and amp.
The Topping pre90 that I now use in one of my systems also has no sound to it. However, it did not play nice with my CODA #8 amp though it sounded great with my Benchmark AHB2 monos. The kicker is that it only costs $599. Do not let the low price dissuade you. You can get this on a home trial and see how it works with your gear. I sold the HPA4 and kept the pre90 because I wanted to raise some cash and I no longer need the headphone amp in the HPA4. The pre90 is very limited in features but in terms of sound it seemed identical to the much costlier HPA4.
I also have a CODA 07x preamp that is not super neutral as the other 2 since it adds a warmish tone to the audio signal. It has become my main preamp in the office system. This preamp would mate great with a Class A amp however, it is not in your price range.