One is supposed to sit on axis with the MID DOME, not the bloody tweeter....
All this and you still use an SCA2? That could be the log-jam if mine was anything to go by, as the entire time I had it (two years from memory), it sounded like a Naim preamp in dire need of a HighCap supply. When you've a few hundred spare, try a vintage Levinson ML28, which is balanced from input to output and sounded rather wonderful I remember at the time.
By the way, the Belden tinned-copper "banjo-wires" ATC recommend are actually quite good, if a little "safe" and treble monotonic sounding. Anything which sounds dramatically different to these should be viewed with extreme caution IMO. You have lifted the XLR pin 1 connection at the speaker end, haven't you, as this sits on the sonics as well if pin 1 is connected at both ends, despite ATC fitting a 47 Ohm resistor to the signal-earth connection in the speakers' amp packs to prevent ground/hum loops.
I also found that copious use of cable ferrites on the mains leads (or possibly use of Mark Grant mains leads today) helped bass reproduction. This was repeatable and three of these won't cost much more than a tenner.
By the way, you have hundreds of feet of COPPER in your voice coils and using proper balanced connections should dramatically minimise the need for exotic wire materials, especially as all the impedances are properly matched, unlike most single-ended phono connections around and about.
Lecture/rant over...
I can't go on and I can't go back, I don't feel so matter of fact,
I tried and tried to make good sense, What's the good to try it all again?