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sorry i didnt get back to you guys who kindly replied to me, we had a few covid issues so ive not been about, but thanks, i did find what i was looking for in the end with your/s help.

I am currently re capping a NAD 2340PE, giving it a general service etc and i noticed as i was changing some of the electrolitics, the voltages were higher than what is stated in the service manual, so i know this shouldnt be a particular issue as the uf values are correct, however i noticed as i was checking all the poly ones, that some of them are the wrong nf values,and also, according to the service manual should be ceramic, so not only are they the wrong type but also the wrong value.The caps are located in the tone circuit(im not yet any kind of expert), so i guess the question is--is this a problem.?? i have the amp up and running but i have noticed a real harshness at some of the treble top ends.could this be the caps? many thanks

At "low" frequency, I don't see any problem using a lighthly larger capacity for a cap, if voltage is respected. Could be another story at high frequency though, or for some very specific tightly trimmed precision circuits. (There is obviously no problem of polarity).

In general, you can blindly substitute a ceramic cap by a poly cap ... as long as you have room (size, volume) for the larger, in size, poly cap. Although, SOME poly cap don't tolerate heat, even have to care when you weld them to the board. For those, you need the same procedure than for thermal fuse (that a standard welding would ... trigger, and kill).

There are, from time to time, a shortage of multi layers ceramic caps, so, it is not really unusual to see the manufacturer having to use a lightly more expensive poly cap as substitute when their supplier was unable to deliver the required ceramic caps... if the volume-size is not a problem (won't work for many cell phone, as example).

Maybe worth of mention, the radial (cylinder shape) poly cap have more inductance than the "thick sheet" ones. And the ceramic caps are more sensible to heat (their capacity changes with temperature) and may show piezo-electric effects (mechanical vibration producing electricity and so, could add noice).

is it possible,if all the caps are the originals, and they are the right make, that the manufacturer would have used a higher voltage for what ever reason(ie some are 25v when the service manual says 16v)?. Are the service manuals ever inaccurate? in thier information, or can you always assume they are always correct and its possible others have worked on the amp over the years but actualy used the same make as the original?

many thanks