I have seen similar posts but didn't quite understand so I will put the question simple and straight:

- Can I just unplug the Bose amp and install a new one with existing plugs without cutting wires and without accessing the console unit?

 

Simple and straight: no.

 

Ok, I went to a shop and asked them if they could replace the whole system (amp and 6 speakers) with a high quality system. The guy there seemed very honest to me and he said that if I did this, it would be a waste of money because the head unit is a bottleneck. The audio system cannot provide the desired output to the amp and the amp wouldn't give the maximum out of it. Adding a high-end amp would only give me higher decimbells but not better sound.

He adviced that the best thing I should do is replace only the rear 6x9 and front speakers. Tweeters shouldn't be replaced because quality wouldn't change much.

Any experienced guys here have a different opinion or is everything correct?

 

That sounds right confusing, what this guy is telling you. With the bose you have to replace the amp and speakers to get good sound. Ear-level tweeters are a must for the best sound unless you have ears in your legs. Bose equalizes (ruins) the signals coming in, and the speakers are designed around that equalization.

I've dealt with 2 bose systems, in a Mazda6 and (currently) a Mazda3. Both have had flat line level signals coming from the stock deck and simply needed connecting directly to a new amp. I can't imagine the RX8 being any different (mine was non-bose and I never upgraded it).