I previously sent a Quad 909 amp to Quad for service, and one of my requests was they replace C7, C9 and C11. They did this for me but using the cheapest possible capacitors and the amp sounded dreadful. These caps are at a critical location in the signal path and frankly i don't think Quad have any clue when it comes to quality components. I will replace these again myself this time, and will use some quality Nichicon for the job.
Quad are good for finding faults and identifying problems you may have with your amp, but if you expect them to replace some parts and your amp to end up sounding better, you'll be disappointed (unless your amp is 20+ years old and then any basic cap replacement would yield an improvement).
Whats the problem with your amp?