When I first got the ATCs I was in love with their midrange. Later, the unimpressive bass (not very extended or weighty) coupled with an uneven top end began to grate. No matter how I tweaked the sound with DSP, positioning etc they left me wanting. In fact, when I sold them to a guy (who wanted them for monitoring in his impressive home studio) he was able to analyse their performance with a Trinnov processor and mic and the measurements confirmed their lacklustre performance. I could see he had been hoping for more (from the great ATC) but in reality they were barely better than his outgoing speakers.
funny ... it seems i am on the same path like you
sold my ATC SCM 40 and now waiting for 4351B to come and will connect them to mini DSP SHD studio
ATCs insist that they rely only on classic and proven technologies ... and are not alone especially in UK (harbeth , proac, grahamaudio). So, is not bug is a feature
But the technology is developing and this world-famous ATC-midrange is just not that unique anymore
On the other hand those speakers cant still be very pleasing for a lot of listener. For me - i found the genelec mids more revealing and fullbody and with more impact and dynamic. Regarding bass and hights will say they are diffrent but not that the one is better than the other. Actually in bass perhaps ATC is better resolving, but is a standmount after all and closed box. I messured the bass-driver allone ... pretty unque parameters. The tweeter is also remarkable linear and low distortig till 0,8-1Khz.
In general - the ATCs are more gentle and calm. Never really enjoyed very dynamic electronic or hard rock music on them, but very nice for a soft jazz, vocals, world.
This is also a good remark on them
https://www.audiosciencereview.com/...tors-to-hear-reverb.30183/page-5#post-1313364
Obviously ATC are also still good enough for studio - quite common and desired.