From what I understand its 4x40 = 160 watts. Most decks for 100 bucks now produce 4 x 50 = 200 watts..
This system is 12+ years old and paying anything over 50 bucks for a speaker is a complete rip off. You can buy two brand new Infinity 2 ways for less then $150.
These systems do last a long time and I give LEXUS props.. I suppose not having the amp in the deck reduces heat and helps with the life, but the cost of a speaker or anything else is pathetic. A new 10+ disk CD player is like $150 bucks, new decks with MP3 capiblity is like $175, 4 - 5.5" speakers is about $300 and then installation would be about $250+. This would be a system that would destroy the 13 year old Nakamichi system.
Recently I took apart the door and compared the quality of the speaker to other 3-5 year old speakers. Everything for the most part was better then the one way speaker.
I dont see how your speakers were failing? Typically the speaker would pop or sound like crap, but I dont think they just go dead. Usually that is a channel in the amp or deck. I would look at the bill and figure out why different prices and what was done?