Carbon film resistors in the signal path are to be replaced by 1% metallic film types, with equivalent value, of course.
Perfectionists select these in pairs, in order to keep channel divergences as minimal as possible.

Which manufacturer to prefer here is not to say, as for the very fine sound nuances you may possibly hear in direct comparison the price differences are disproportionate to the 0.1 % types.

Out of all capacitors the M - Cap Supreme from Mundorf or, Musicap from  Hovland is indeed the supreme one which I can confirm myself due to intensive comparative sound hearings. Unfortunately his measurements forbid to use it in the most places. The increased price is fully justified by the complex manufacturing process and by the result of its sound! "No capacitor has more depth and more refined nuances of the musical events than M - Cap Supreme or Musicap".

You'll find this capacitor nowhere else except in a special 50.000 $ loudspeaker.

This capacitor is also especially suitable (in 0.1uF + 1uF version) for the parallel connection of electrolytic capacitors in the current supply. The current supply can be delivered substantially faster to the very benefit of the dynamics.

However, if you use this capacitor within the frequency switch of a loudspeaker you might not believe your ears: it seems as if the musicians were playing live in your room!

You don't hear the only hear music!

About operation amplifiers one could fill volumes, but all play their cards close to their chests.

This is acceptable since one cannot call a certain type as "the best for everything", it rather depends on the circuit, function and use. To find the best for each is a very demanding task that needs a lot of time measuring and drawing sound comparisons. So one really can understand why the manufacturers, technicians and tuners keep their cards close to their chests! Even I don't want to put my cards on the table. But on request, if you tell me what operation amplifier you want to replace, I'll gladly send the best back-up version, at cost-price!

That much I can say: Burr Brown operation amplifiers ICs are leading as far as sound is concerned. They are capable of transmitting frequencies without any loss which are about a hundred times faster than a music signal, at least if you use the ones which were designed for military, medical or space technological purposes. But don't use sockets as they again could reduce sound quality!