First, keep in mind that any newly installed capacitor takes time to break in before you can make your final decision. From what I've read, the PIO or Vit Q types take a little longer.
With this old tube gear, I factor the high internal operating temperatures into my decision. The 96P or 196P series of Sprague's Vit Q Caps are rated to 125 C.
"Vitamin Q" is a proprietary oil Spraque used in their hermetically sealed Paper in Oil (PIO) capacitors. That's it....the oil.
Other companies made very fine PIO caps, West-Cap is one of my favorites. They made a CQR series (Polyethylene in oil) that I was bowled over by. Nothing added to the sound, crystal clear in my opinion.
To clarify that statement, I'm going to use the analogy "All Glass stops the Wind, but how it lets the light through is a different story". In this analogy, Glass is the capacitor, Wind is the DC being blocked by the capacitor, the AC music signal is the Light passing through. As we all know, different materials have different refractive indexes, look at a stick in the water and see it bend.
There's your "quality of sound" analogy.
Where the capacitor is going and what kind of voltage it will be seeing must be factored into your decision. Some caps are considered "bright", some "tuby". I use these descriptions sparingly as a guide. I wouldn't use a capacitor construction considered by others to have a "tuby" or "dull" sounding in a phono stage, especially not the RIAA.
Some caps are built with extended foil to reduce inductance, some are affected adversly by temperature.
As Larry said...many opinions exist whether or not a capacitor imparts it's own sonic signature. It seems that many, but not all Electrical Engineers are of the opinion that a capacitor is a capacitor and nothing more. Based on personal experience, I feel differently. No disrespect to you Larry or what you wrote, this is a discussion I have had with other engineers. I try and keep an open mind regardless of what I've been taught.
I have only used one modern "boutique" capacitor in a phono preamp and thought it was spectacular..Mundorf's Silver in Oil. I had to try based on what I read and was impressed. But that was in a solid state/tube hybrid phono stage that doesn't get very hot at all. Besides being ridiculously expensive, I doubt they could last very long in the hot internal environment of vintage tube gear. They're also huge, so Real Estate enters into the equation.
Cost wise- The NOS vintage Vitamin Q genre aren't as expensive as their reputation suggests. I think $5 to $10 per cap isn't that unreasonable, all factors taken into consideration.
Last point, I got the impression that you may have changed a few values from the original design. I would suggest you first start by replacing any coupling/blocking or decoupling/bypass cap in question with the original value and use a construction similar to the original construction. For example, "Black Cats", "Red Devils" are PIO types not hermetically sealed. I would stick with a PIO cap in those positions.
In other words, go back to square one as best as you can. That way you can truly compare apples to apples. Even though the bottom line was considered, Fisher's engineers made their choices for a reason.
The Fisher Brand was successful and around for quite a while. This could never have been the case if their equipment didn't have a sonic edge over the competition or if there were reliability issues. Success and Longevity are two factors that led me to Fisher in the first place. I trust their engineer's designs.
