cables are they worth it?

In short, no - an $800 interconnect is ridiculous. Likewise for a $50 power cord. I am sure someone would sell an $800 doorstop also - because it will "just hold the door open better". THere is one huge advantage to expensive cable and power cords - it allows for large profits. Good old Belden cable or any other reputable big name manufacturer will work just fine. You can terminate your own interconnects for that matter. Look at the cables used in other areas of electronics that either deal with low level signals or feed power to critical equipment. The power cords on ventilators are nothing special and they keep people alive. The interconnects on biomedical research and clinical equipment are also not particularly special - I worked in that field for decades (background is EE). What you are sending between your preamp, amp, source etc is just information - it isn't even in a form that is all that difficult to deal with - it's rather high level (certainly not in the microvolt range) the frequency range is not difficult to deal with, you don't have dc offset problems. Don't you think that information that determined patient treatment would use $800 interconnecting cables and expensive power cords if there was a reason to. (Some manuf did use lo leakage pwr cords for electrosurgical units - but I doubt that you are going to use your amp for any surgery in an OR)If you are worried about induced noise, what is important is whether you have a balanced connection or alternatively, a properly grounded or common driven shield as the case may be. That doesn't add any significant cost. As to a power cord - there is just no reason to not use any power cord of the correct wire size. A much more important area to consider is providing a good ground for your equipment.

the amp (....) is a Symphonic Line RG9 mkIII

Right. I once had a RG 10. The spkr cable I then chose was Nordost (the bluish-purple one). And single-core unshielded for my sources -- EXCEPT the TT of course.

Musicnoise,

An audiophile was boarding a plane.

"Excuse me sir, but you will have to run that briefcase through the X-ray inspection machine."

... a moment later the secuity inspector frantically hits the emergency stop button on the X-ray machine, as he sees the audiophile lying down on the conveyer belt about to enter the machine.

"What are you doing - you can't put yourself through that machine. Just give me that suitcase and step through the metal detector over here."

"I can't, says the audiophile, it isn't a suitcase at all - it's the power conditioner for my pace maker!"

Borrow some good cables to see for yourself. Only you can put a value on the change in resolution.

Dave