You can spend a lot of money on exotic capacitors, resistors, switches etc.
Gold/Silver paper in oil capacitor?
Pure Silver wound transformer?
How much money do you have sir?
I recently had a preamp returned for a modification upgrade.
It had been filled with expensive capacitors , silver plated switches and ridiculous amount of cap bypassing, along with some questionable soldering skills.
Unfortunately the "technician" who did the work had also made the preamp un-operational and it was returned to me for a repair and upgrade to latest circuit.
Over the years I've tried lots of premium parts in my preamps, and there is definitely a place for the best quality possible, some components are very critical - transformers for example, and my distaste for electrolytic capacitors doesnt have to be repeated again- Supratek preamps have about half a dozen electros at most while most preamps have hundreds of the nasty devices.
For signal capacitors I use polypropylene film or good quality paper in oil caps, yes there is a difference in quality between a cheap and nasty polyprop , but once you get to a certain level of quality , differences become just differences, not significant steps in quality.
And these differences are quite minimal, mostly inaudible, although the cost of some of these items requires the brain to justify some audible "improvement".
I left the expensive caps in the preamp when I rebuilt it, at the customers request, but on listening to it I had to take one expensive pair out, as they were just horrible sounding.
This preamp did sound different, albeit it was minor, compared to the stock preamp built with my choice of high quality, but non exotic components.
It certainly wasn't "better" in terms of sound quality, it had a slightly different tone, very minor, but probably enough for a zealot to proclaim as significant.
The fact is changing the current running through a tube, like the 6SN7 for example, from 4 to 8 milliamps will make an audible difference, changing the operating voltages, or the type of plate loading will make a difference, as will an output transformer designed and optimised for the application.
Most of the time these changes can be measured (not all the time) - the role of the circuit design is paramount to sound quality, not the level of exotic components.
Good circuit design, over engineered and high quality components ( not necessarily overly expensive) are far more important than filling a chassis with so called exotica which has profit for the vendor as its main purpose.
As an aside, I do like a Russian military teflon paper in oil cap I use in my preamps as it seems to have a nice synergy with the circuit and other poly caps in preamp.
Its not cheap, but a fraction of the cost of an exotic Teflon capacitor that frankly sounds brittle and nasty.
