I Recapped my Conrad Johnson PFR. Wow! Some of the best $$$ I've spent

The first mod I did was put a Noble potentiometer in a Superphon preamp, many years ago. Big difference! Been a modder ever since.    

Ask any tube amp designer/maker about any of his products which have been damaged by well-meaning modifiers who didn’t understand why certain parts were used in his product, and who replaced parts chosen for a specific reason with inappropriate parts. A boutique part may have electrical characteristics (unknown to modifiers) which makes it unsuitable for use in a certain circuit. I have heard horror stories told by a few of them, including Bill Johnson, Frank Van Alstine, and Roger Modjeski.

@bdp24 No doubt some truth in what you say. However, manufacturers must bear some responsibility for being cheap and using inexpensive components. 

Sometimes a more expensive part is a better part. At other times a more expensive part is just a more expensive part. 

@bdp24    In my case I wasn't try to modify but  trying to save a good sounding preamp which has run its lifetime on the caps. My tech (Bill Watkins jr) just uses these caps because they are good...at least that's my understanding.
Still, I understand your concern. I suspect design concept is probably a balancing act as much as anything. Symmetry comes to mind...just like putting together a good sounding system. Mfgs do it in the micro. We do it in the macro. I have my systems synergy worked out after years of effort. I thought I was going to have to replace the PFR because it is old (Still may have to do so). So its nice to keep that symmetry...even though that tube preamp looks awful tempting......