Nick, I think you've gotten some good advice on this question on AudiogoN, but here are my thoughts.
Replacing EVERY resistor and capacitor in these 3 pieces sounds like gross overkill to me. I'd start with the 'passive' components in series with the signal. Then maybe I'd replace (if the piece is tubed) or seriously bypass the PS caps or maybe add some decouplers/bypasses close to the amplification devices. Then I'd replace the parts that ground the signal, ie load resistors. I suppose the designer recommends replacing ALL passive parts because he/she is aware of the continous quality-v-cost decisions he/she had to make, but there ARE parts in these that make more difference than others. Having schematic diagrams would be very helpful.
Michael Percy here in the US sells the very fine Holco resistors for 45 cents to less than $2, so you should be able to find these English resistors less expensively (if I'm correctly interpreting your '£1200' comment*).
Passive componenets do indeed have a sound. I believe that EVERY change to an audio system changes its sound; whether you or I can HEAR those changes is another question. Can you easily hear the differences among IC or speaker cables in your system? Or are you more like me in that you have to work hard to hear some of those differences? I believe that the degree of change in sound caused by changing a couple passive components is roughly equivalent to the sound changes caused by changing a pair of similar-quality IC cables. Of course, some of us are more sensitive to some kinds of changes than others.
Perhaps you could let us what know this equipment is, its approximate market value, etc.
* Had to find the Windows special-character map for that one!
