ARC Ref3 or McIntosh C2300/2500

@kalali

No material difference between 100K or 1000K input resistance. Both are high enough such that any stray micro-amp current could generate a voltage input to the amplifier.

You need to get to 10K or below input impedance to start making a difference. This means the preamp needs to have a stronger drive output - all good stuff as anything that increases the need for a stronger line level signal means less noise as the end result.

Very old gear used 600 Ohm as standard and was immune to the kinds of noise now affecting high impedance gear but that standard actually came from transmission line theory for telegraph wires so it had other issues.

Another solution to noise on input is to go balanced (this means each signal wire to ground should have the same impedance) - this ensures that RF/EM hum/ground loops are affecting both wires equally and cancel out. However even balanced works better 10K and below for noise suppression.

So as a general rule, ultra high input impedance is a bad design if you care about noise. If you wanted to deliberately design an amplifier to detect very low level signal noise then I can guarantee you it would have a very high impedance just like an oscilloscope input.....

Very good explanation shadorne, and I would agree completely. 
Whichever path one chooses, I believe it is best to have the Pre Amp and Amp from the same manufacture and from the same era for optimal pairing and experience the complete sound that each manufacture has to deliver.  That being said I went from an Audio Research LS26 and Audio Research VT100MKII, to now a McIntosh C2500 and MC402 ( also an MC275 tube amp ).  Many of you would say that was a backwards move but my experience was this:
Both sound very good, the ARC combo is probably "better" in that it is more detailed were as the McIntosh is a bit "softer" and "creamier" if that makes sense.  So why did I switch?  Because I found that with the ARC I was only able to enjoy a very few pieces of music.  Ones that were very well recorded and with the McIntosh everything sounded good.  My music preference is mostly old classic Rock recordings of which many are not the full high resolution of some of the best audiophile pieces out there.  example old David Bowie was un listenable on the ARC but sounds wonderful on the Mac.  But again an Audiogon Jazz Sampler in Hi-Rez was better, more detailed on the ARC.  Also with the additional power of the MC402 to my Wilson WP6's, it has dynamic headroom and "punch" for days.  I of course would love to hear a Ref level ARC pre/amp but also it would cost more than twice as much.  
Also recently added Shunyata Sigma NR power cables to the McIntosh and it takes it to a whole new level of 3D sound stage. 
Just my experiences 

Ref3 with out doubt. I just moved my Ayre KX5 on and kept the ref3 over it. The KX5 is a fantastic pre , its just the Ref 3 is a little better.