improvedsound- you’re in the deep end, so take care.
My knowledge is anecdotal and experiential. I have experimented with foil inductors enough to learn that I’m over my head. Indeed a foil inductor is a more perfect inductor than any wire inductor. However all components exhibit all characteristics ie, an inductor also has resistance and capacitance plus the products of reflectance interactions between them - and time factors as these interactions play out. So, changing anything changes everything. Flash back to my direct observation of Jim’s learning curve about these matters, as each iteration of each product grew into its maturity. Part of that advancement is about understanding and implementing more aspects of such component interactions.
There are direct aspects that can be adjusted. A foil coil will have much less series resistance and different capacitance characteristics in space and time per equal inductance value. In some cases those changes can be corrected via addition of a series resistor and/or layout changes to compensate for circuit capacitance and resonance variables. I have been able to accomplish some of these requirements in my experimentation and coaching. However, I’ll say it again, I’m a novice and not qualified to re-engineer Jim’s work beyond basic changes.
So I keep the extant inductors because I know them to be best-of-form (at any price), providing an anchor for any other circuit changes. Similarly with caps, ClarityCap provides very predictable performance in all measured aspects - it’s no accident that they are an industrial / aerospace / professional company making products based on solid engineering performance. (I have tested and measured some brands that de-spec some aspects to achieve some euphonic outcome.) A person could spend a lifetime making sense of the correlations. I’m keeping it simple enough for me to make sense of it within my constraints.
Regarding resistors - the Mills sound better than Jim’s. Fair enough, they cost 5x as much. Jim developed, from first principles, what is called the Ayrton-Perry winding which is non-inductive with minimal parallel capacitance within an inexpensive sand-cast case. His circuits and layout assume those A-P characteristics. I know that Mills MRA-12 is a direct drop-in with the same characteristics. However, I don’t know much for certain about Path, or film or bridges. I do know that altering the type of resistor or any component will have effects that are sonically important. I also know the hundreds, perhaps thousands of hours learning by listening and integrating with fundamentals of physics to get the holistic design expressed in each product as designed.
I do not have the knowledge to help unravel what might be going on when some component is changed. I do know enough to be extremely cautious and make changes that don’t screw anything up. I do not deny that some folks have gotten results they are quite happy with, but the system is complex enough that a dose of luck could be operative. So for myself, caution wins. Trust your ears. If something sounds "off", call it off. Then the fun begins putting Humpty-Dumpty together again.
This very long answer is to recap territory we’ve explored in these 223 pages over that past few years, which new participants probably haven’t read. Keep the faith, learning is good.