Metal or Carbon Resistors - Which is better?

EMIA attenuators use the same autoformers as Intact Audio.  The EMIA products just put the autoformers in a nice looking box with remote control.  Intact Audio sells the raw autoformers for the DIY market.  Moreover, the autoformers all sound the same regardless of the number of steps.  Well, with one big exception—-there are both copper and silver models.  All of the copper models sound the same; all of the silver models sound the same.

I have the 47-step model and there is a small 1.25db difference between each step.  In my system that easily spans the range I need for my various sources.

“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.”

+1, @larryi 
Exactly. I am trying to see if I can keep the signal (from TT) with minimal coloration. This is my 2nd attempt to access and possibly keep a 300B amps in my system. First attempt was a complete disappointment disaster (please don’t ask about the amp manufacturer). The plan is to audition few well designed 300B amps before settling on the ‘one’ that highlights the true virtues of 300B valve known for! 

The passive pre under consideration is a simple circuit, zero flash, all substance. How good it sounds, only time will tell.  

Here’s what I know about passive pre design, a passive volume and balance control using a 23-step rotary attenuator. The signal path passes through only two fixed resistors at any volume setting, minimizing degradation. Gold-point, double-contact rotary switches (the kind used in industrial test gear) are used for both volume and the 3-position input selector. 
 

@grannyring  - PM sent! 

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?

@larryi 

Appreciate the thoughtful take and advice. I wouldn’t be here if I didn’t keep an open mind :-) 

Metal better for audio applications. Look it up.