Whatever solution you use, there has to be an amplifier (integrated or pre+power), whether it's seperate or build in.
If you wish to keep your current CD and Tuner, then a replacement is necessary. To get advice on this, then more info is necessary ie. What was your out-going amp, what make is your CDP, Tuner and Speakers and what do you want to spend.
You haven't mentioned budget or speakers, but the new Rega Brio-R is one of the very best amps around in its price range. At a later date, the Rega Dac can be added, which again (imo) is the best in it's price range.
The 640A retails at £350 at the moment. A repair job will probably cost you £50 to look at it, then depending on fault parts and labour on top. And don't forget to factor in courier back and too, if you don't live near a decent specialist. No idea what's wrong with your amp, but I'd be surprised if many faults could be sorted for less than £200.
As such you'd probably be better off getting something else. As mentioned, check out second hand, your money will go much further.