I have both. Actually CA840a is v1, so that might be slightly different.
Teac is really smooth, fast, enjoyable. Everything sounds right - any genre. You almost never miss the tone controls. It is just fun to listen to. Still the chassis feels i bit cheap. Unlike ca it has phono stage.
CA feels like better gear physically. Sturdier. But it is probably bit overengineered with relay volume control etc (mine still works fine), but you also get great input flexibility. Sonically it is more detailed, but i am still not sure if as much fun to listen to as teac. But you get tone controls option.
Both can be used as power amp for avr on one input, which is great.
I compared them on german Elac 607 speakers, which are probably level or two higher (10k usd originally) - very precise and detail rich, tight with aluminium/mdf chassis. Using squeezebox transporter as source (lossless cd/tidal or hd material).
I also tried vincent 234 amp. Built much better than either, incredibly detailed and refined, but with elac speakers good only for narrow selection of genres. Bought by B&W fan, who is totally thrilled by it.
Still undecided which to keep. Probably teac and eventually step up to accuphase or yamaha 2100.