Hello Artrix,
I feel your concern is genuine but the only way to really get a convincing answer is to find out for yourselves. However, I can share my thoughts on this, but it is only limited by my taste, beliefs and experience. I guess that what you wanted to know, a few different opinion from techs as well as other owners. ( I am both) This is going to be a bit long.
A complete restoration does not mean upgrades. The restoration means doing whatever it takes to get the deck to work to factory specifications. Apart from replacing some rubber parts like belts and idler tires when required, the restoration involves a combination of mechanical and electronics adjustments using dedicated gauges, test tapes and measurement devices. Setting one thing correct will require almost all of the chain to be readjusted. There is nothing called a single fix in restoration.
If the track height is adjusted, The azimuth also will need adjustment. When both these are changed, The L/R playback level has to be re calibrated. The playback level change will require recalibrating the HF peaking. When all these are changed, The recording level and bias for each type of tape also needs to be re calibrated. All these require the meters to be calibrated in the first place.
If there is a problem, it can be fixed. That is not restoration. Whether you need a restoration or not boils down to the purpose of service. If the purpose of owning a Porsche Carrera Gt is only to take it to office everyday and show it off, yes, just an oil change will do. But if you want to get it beat 0-160Km/hr in 6.9 Sec, as promised by Porsche, it may take a lot more work plus the driver should know very clearly what it means him/her.
In order to do a restoration you need a fully functional deck. So making it fully functional is the first step. Whether you will need a restoration or not is something that depends on how the owner feels and concerns about the following
Am I hearing the songs in their correct pitch?
I am I hearing the bass loose or it is the wow and flutter of the tape ?
Is my deck reproducing the finer details like the high frequency air, reverb tails and the sustain of bells etc properly?
Is the sound stage and panoramic position of various elements accurate?
People who just enjoy casual listening of music normally will not have these thoughts at all. For them mp3 playing from a media player, a song playing on a TV, CD, Tape, LP, all are the same. They are merrier listening to the songs as they get to hear it. But if the owner is not in this category he/she may get possessed by the audiophile ghosts (they are a big family). The analog tape ghost evokes thoughts like the ones I listed above and haunt them. The only way to keep the ghost contented is to do a restoration.
Coming to hi-end capacitors, in nakamichi decks some capacitors (the orange drop types) fail. Some of these are very accurate 2% types. They are there for a reason. The failure happens after 20 to 30 years from production and the rate is more on decks which have been unused for long. The 2% types replacements are expensive and difficult to obtain relative to what is normally available. It is better to replace all of these if one has failed. Some techs replace all of them regardless as a preventive measure. Not all Nak models have them, the RX505 do. I normally try to educate the owners (not to convince) and leave the choice to them. If the cap is already failed I certainly recommend replacing all. This is just because they will fail invariably as it has started. Moreover the component cost is only a small percentage of the service charges. So it is better to replace all and save on repeated visits to the tech. No tech would prefer this as it affect their reputation too.
Also the electrolytic types do change value over time. Only if they have changed Loss of HF, increased noise and more THD are the results (this is assuming that the transport tape path and the head and rollers are correctly aligned). Only if they are bad they need to be replaced and All of them need not be replaced with audio grade capacitors. Only some which are on the audio path, especially the ones marked in the service documents as LN types needs audio grade types. Normal ones can be used but it will result in more noise and signal to noise ratio will go out of specks.
In performance, I find replacing these with audio grade capacitors with hand matched values in L and R, the sound stage is greatly improved. But this means only to the people who are haunted by the sound stage.
Actually the build quality of audio grade caps eg: Elna Silmic II or Nichicon Muse are very high and they last very very long time compared to the normal type (value shift over time is less) So the extra cost is worth in a non audio point of view.
Changing the opamps etc are only a matter of personal taste. The original 4558 have BJT inputs and lower slew rate. So the sound is generally warm and have little extra harmonics distortion on the lower bass range which give a deeper but softer bass. The NE5532 has JFET inputs and they have tighter bass and crispier highs due to higher slew rate. OP275 is an angel from heaven with extremely low distortion and unbelievable slew rate. But unless one has the amps and speakers that can translate this level of sterile sound, the ears to listen to it and more importantly the content to exploit this, they may not hear the difference. Being in the service field of professional studio recording equipments for the past 12 years, I see that 90% of the equipment used to record and mix in studios use NE5532 or TL072. The brands which use BB opamps are un affordable even to most of the studios. But with content from Audiophile labels like Chesky records, Blue Note etc, the difference is very much noticeable. ...Well...may be only to the haunted....
