Metal or Carbon Resistors - Which is better?

I suppose it would also matter which specific brands were offered too.  A local builder of very high end custom electronics likes certain vintage carbon composition resistors for the rich and warm sound he favors.  Most of his builds use the likes of vintage Allen Bradleys, for higher builds it might mean vintage IRC resistors, and so very special builds employ Western Electric parts.  

For some passive linestages, the resistors are almost the whole game—they have a few switches/selector, and attenuation means a stepped attenuator or relay switching of different combination of resistors.  The right choice is important.

Transformer/autoformer attenuators do a better job of preserving dynamics and sounding vivid than other types.  Instead of burning off the signal in a resistor, transformers reduce voltage by converting it to increased current.  Because of complexity, transformer volume controls have a limited number of secondary windings, and hence, limited number of volume steps.  Autoformers can offer more steps, and some also offer convenience of remote control and balance adjustment.  A good example, is the one made by Emia.

I know you are trying to minimize coloration by utilizing a passive linestage, but, ultimately, the goal is to achieve the best sound by assembling good and compatible components.  The challenge with passive linestages is a loss of dynamics—the sound tends to be less lively than that of good active linestages.  To me, this would be particularly of concern with 300B amps which also tend toward the polite and undynamic side as compared to 2a3 and 45 tube amps.  
 

I would not narrow the field to any particular design or approach.  While I’ve heard some good passive systems, in most cases, I’ve preferred a good tube active in direct comparison even when the system was theoretically passive compatible.  The active system tends to be more dynamic, harmonically rich and saturated and more engaging—are some of these qualities ‘colorations”?  Perhaps, but who cares?