new at this

I have B&W htm1 center,cdm-nt9 front, cdm-nt 7 rear. Adcom 7500 (5 by 150) amp and Adcom pre amp. Mids and highs are harsh from mid volume on up. Could this be due to cheap rca cables? ( I have Radio Shack's best!) Front and center speaker cables are big....the size of car battery cables. The wires inside look to be about 14 guage. I paid about 5.00 per ft.....not sure what brand. Rear speaker cable is Monster 14 guage. Front speaker cables are about 8 ft long....rears are about 25 ft. Music source is usually Comcast Cable digital music channels or 250.00 Denon CD player. So, is it cheap rca's, cheap speaker cables, all of the above, or none of the above? There are ton of cables with a 5 channel system and I do not want to spend a lot of $$$ and not hear the difference. If new rca/speaker cable is the answer, what would you recommend?
Also, I have seen power cables discussed here. Is that the power cable from the amp to the wall outlet? Am I to beleive that matters too?
I asked these questions at my local high end store and was told I need new cables. Another said I need a new amp, possibly Rotel, and went on to say that beyond Radio Shack sheilded rca cable I would not see much improvement. I am forever the sceptic when talking with commissioned salespeople! I would like to know what people who play with sound systems for the love of it would have to say. Your comments are greatly appreciated.
John