Just went through some resistor comparisons between choices for Khozmo and Hattor preamps and my SMc buffer. I have not tried the AN Niobium resistors.
In my Hattor preamps, I tried the Takman metal film and liked the sound but I actually liked the Amtrans AMRG Carbon Film Resistors better. They just sound so musical in the Hattor Big preamp compared to the Takman REY metal film resistors in my Khozmo passive, which also sound good but more precise. I have not tried Takman REX carbon resistors.
In my SMc buffer/preamp, the volume control (that I removed from the circuit) used AN Tantalum resistors, which were very musical with a smooth groove. I replaced the VC in the circuit with two Audio Note Silver Tantalum non-magnetic 2W resistors . Those AN silver resistors sound very nice having body, resolution, and musicality. In summary, my favorites (of what I have heard) for preamp use are the Amtrans AMRG Carbon Film Resistors and the Audio Note Silver Tantalum non-magnetic 2W resistors.