AN Niobium versus Tantalum resistors?

your results with full Z-foils mirror my own. Ideally I use a mixture of Z-foils and Takman metals, with a few Caddock MK132 (very full, bold and assertive) and Shinkoh tants (musical and finely textured) thrown in for flavour.

The MK132‘s are very good at balancing the  Z-foils in circuit.

I recommend replace one position of either the Takman or Z-foil with Niobium. I tried a few combos of this.. small differences of these locations, but in all cases the improvement of Niobium was great!

I will try a Niobioum on the input position for 500 hrs

I replaced the VC on my preamp with two AN Silver Tantalum resistors, turning it into a buffer.  The AN silver tants are huge in size and while maybe not exactly neutral they sure are musically enjoyable, IMO.  I have an outboard passive VC with Takman metal film shunt resistors and Vishay z-foil series resistors and it is comparably a little hard (but neutral) sounding compared to my other outboard VC that uses Amtrans AMRG Carbon Film Resistors, which I find to be a good combination of musical and detailed.  I doubt I would gain anything by replacing the AN silver tants with AN Niobium, but it would be easy to do.

It is Interesting how incredibly subtle aspects of complex field expression (current and voltage), as combined with molecular noise, all time shifted in some manner, as a slur or as a displacement in time and level of transient edges (the only part that matters to the ear--it is all we hear!) (that takes on aspects of a hysteresis curve, in written graphic expression) can combine to alter our understanding of an audio signal reproduction.

Where, knock on wood, the haters have yet to arrive. To speak, and share, without receiving undue abuse.