Carver TFM 45 or Cambridge Azur 851A

First, it will help if you provide information regarding the rest of your system and room. I.E. speakers, room size, how loud you like to listen, and any shortcomings you perceive.

Second, you would be looking at the 851w, as you only need an amplifier, not an integrated amp, as your 851n is a pre-amp/music streamer/dac.  

As far as the TFM-45, it was a decent amp in its day (some will say horrible, some will say great). That said, it is over 25 years old and internals may have aged. A newer amp will likely sound better, less grain, and more clear (the analogy being like cleaning a dirty window).

There are also many choices outside of the Cambridge amp. At $1500 there are many great choices, especially if you are willing to buy used. If you can provide additional information, it will help others to point you in a direction.

Thanks - The room is about 15x20. It's the living room which opens up to a dining room. The Definitive speakers are mounted in the vaulted ceiling angled somewhat towards the listening area. I listen to both jazz and classical orchestra music at moderate volume. I have no problem buying something used - my main criteria is that it be trouble free for the next several years. 40 years ago I used to work for Marantz and had an 1180DC amp. All of the potentiometers became oxidized and I would up selling it on E-Bay. I don't want to deal with that again...

When you say reliability, the first brand that I think of is Bryston.  20 year transferable warranty.  I would look at the 3 or 4 bsst or newer.  

You might also look at Classdaudio and Emotiva.  Both offer high value for the $.  

Apparently that Carver amp is a very popular item.  One in mint condition just sold on ebay for $430 with 42 bids on it.  There are lots of great power amps on ebay for lots less that will greatly outperform the Cambridge Audio, IMHO.