ATC active vs. passive

I used to have  Analysis Plus full loom (same speaker cables you have) and found it clouded the soundstage. Eventually moved to Purist audio and Nordost for power which was a step in right direction for opening up. Not a fan of AP for power cords on any component. Totally different system, but I think you have many options of tweaking the top end to your expectations. I have found the ATCs to be a very neutral reference. 

Had Heimdall 2 on my Harbeth shl5+XD and found the sound too warm, almost muffled sounding at times.  Audience Au24 sx was much better.

https://partsconnexion.com/speaker-bulk-cable/
For not much money, I believe you can improve your lot.  Also, dpac996 is correct with the Furutech DSP 4.1 recommendation.  Cheers

@ei001h I was looking at the other amps you used with your SCM40’s; The Mastersound starts clipping (1%) around 10 wpc. The Grandinote is rated at 37 wpc and starts clipping (1%) around 30 wpc. Neither amp produces close to double the power as impedance goes from 8 ohms to 4 ohms. The Luxman 595 (very familiar with this, as I used it for many months with Harbeth SHL5+XD) is rated at 30 wpc into 8. The 595 is the most powerful amp out of the three. However, given the Class A envelope and overall low eff of the SCM40’s I can imagine a sound that is not all that stretched out at the extremes. The SCM40s need a wide power bandwidth amp (minimum 75 wpc, better with more power) to show the listener what they are truly capable of. If you don’t change your amps but get the larger ATC’s ( even more power hungry) I am pretty sure you will not get that sparkle and air you are missing.
I am confident that amps such as Luxman L509X/Z, Ayre VX-5 Twenty, Moon 760/761, Boulder 866, Gryphon Diablo 120, etc., would bring you the added sparkle and improved dynamics that I think you are missing with the low power class A and tube amps.

@dpac996

your statement about my amps matches my observation exactly as you predicted

GrandiNote and MasterSound ran out of steam very quickly, I had to max out GN volume to 100% and it was barely adequate

same with MASTERSOUND, beautiful midrange and highs but absent low end

luxman by far the most powerful and I am using it exclusively. 
 

I also have Auris fortissimo tube amp which is 100W, but 595 sounds better 

Which tube amp do you think would do justice to scm150?

SCM150 are serious business :)

I love tube amps. One of my all time favorite tube amps is a Luxman MQ-88uc, but careful consideration goes a long way, as i'm sure you know: room/speaker/ realistic expectations on scale etc.

With the SCM150 and your big room you will probably need to look at push pull tube amps well over 100 wpc. ARC 160S, Carver M350, Octave monoblocks...

I will tell you that I am getting fabulous results with a Moon 761 north amp; The ATC midrange is super special and I'm hearing liquidity and natural vibrant warmth that makes me thinks there are tubes in the chain. I think with any tube amp you are going to be dealing with compromises on big speakers like the SCM150's, but sometimes you can hit on a synergy that defies the paper predictions.