ATC SCM40A speakers

This is definitely one of those situations where you must simply go out and  thoroughly audition both pairs of speakers and let your own ears be your guide.  Happy listening. 

@audiotroy ,

Just came to me you might be addressing @audphile1 and not me.

In that case; correct, sir.

OP,the ATC SCM40A is an ACTIVE speaker..The B&W 803 D3 is a PASSIVE speaker..
Are you looking to DO AWAY with your amplifier & move to a SELF AMPLIFIED(Active)speaker?
 If NOT,then you want to look at the ATC SCM40 PASSIVE (NO "A" in model # ),which are world class speakers with,IMO,a much sweeter treble than any B&W & maybe a slightly richer mid range,though B&W has a pretty decent midrange...

Way back from the SCM40, but have the SCM19v2.

Fabulous speakers. My last speaker purchase.

Ran ATC SCM 40A for 3 years, then gifted them to a friend.  They are fabulous and reveal each improvement made in front of and to them (Gaia ll footers).  
They provide immediacy of attack, space around vocals and instruments, are dynamic and sound stage well.  

They do not like tube preamps IME.  Tubes were too slow.  They can be a bit merciless with bad recordings.  They are not aggressive.  Only revealing.

Replaced them with ATC SCM 50A.  Best of luck.