Neutrality, absence of coloration and tonal accuracy are definite requirements, as is reliability
I would recommend the Parasound Zamp with a ~$30.00 Alps or Noble volume pot. I love this little amp but beware it does not soften the sound in any way and bipolar's are not as forgiving as MOSFET's or some tube systems.
Since you probably won't want to make your own pre-amp and need more than one input you could try the Parasound Zpre but it will color the sound.
IMHO Naim, Musical Fidelity, Exposure. Naim are all colored and not nuetral.