Well seeing as I own Active 100's - you already know the answer!
More seriously, I have not heard the TD12's - I bet they sound awesome and I am not sure what sort of a level up you might get - if you love your TD12's then it might be sideways or even down for you.
On paper the concentric Tannoys have the advantage in imaging (will help close up) but the ATC Active amps will give you more "jump" or deafening dynamics and an effortless sound, as the tweeter and midrange amps are entirely separate from the woofer (much less IMD than passive designs and much higher efficiency = insane SPL levels). Judging by the TD12 plots on Stereophile - I think the ATC's will give you less bass and less thickness in the lower midrange and less at the very top end - essentially the ATC's are pretty much flat and sound that way (potentially too thin for you) - absolutely no sizzle or boom boom just really great mids are what stick out - until you hit higher SPL's at which point they become scary live kind of realistic with the transient impact that will restrict you to only good quality recordings or you will want to the leave the room/run for cover (also unlikely to make you popular with the neighbors or your spouse/GF). Bear in mind, both these speakers are quite similar in the sense they are dynamic power houses that play well at low and extremely loud levels...although the older Tannoy horns are best known for "jump" factor.
More seriously, I have not heard the TD12's - I bet they sound awesome and I am not sure what sort of a level up you might get - if you love your TD12's then it might be sideways or even down for you.
On paper the concentric Tannoys have the advantage in imaging (will help close up) but the ATC Active amps will give you more "jump" or deafening dynamics and an effortless sound, as the tweeter and midrange amps are entirely separate from the woofer (much less IMD than passive designs and much higher efficiency = insane SPL levels). Judging by the TD12 plots on Stereophile - I think the ATC's will give you less bass and less thickness in the lower midrange and less at the very top end - essentially the ATC's are pretty much flat and sound that way (potentially too thin for you) - absolutely no sizzle or boom boom just really great mids are what stick out - until you hit higher SPL's at which point they become scary live kind of realistic with the transient impact that will restrict you to only good quality recordings or you will want to the leave the room/run for cover (also unlikely to make you popular with the neighbors or your spouse/GF). Bear in mind, both these speakers are quite similar in the sense they are dynamic power houses that play well at low and extremely loud levels...although the older Tannoy horns are best known for "jump" factor.