ATC active vs. passive

@lonemountain 

1. is it possible to self modify Active into passive down the line if get tired of active and would like to try other amps ?  Is it possible to purchase passive crossover and swap them at home? 

2. 150 is considerably wider baffle than 50. How does this affect the imaging and the ability to disappear. Logically, 150s should be much harder to disappear 

The OP needs to make an appointment to listen to whatever speakers he decides upon because he has very particular tastes about the flavour of what he hopes to hear.

By all means discuss and consider what others have to say, then leave home and listen, to anything really. Go to the nearest big city and make a trek from dealer to dealer. Tell them your budget and ask if they have anything they think you should hear. Listen. You might find something entirely different to what you envisaged which blows your mind. Or not. Get out and listen.

I have passive ATC SCM40 v.2 speakers in a 22 by 24 foot room, which replaced similar ATC SCM35 speakers. I replaced the speakers after 18 years of ownership primarily because the production of the tweeters was moved in house, and the treble became significantly smoother and less hard with poor recordings. Otherwise, they sound and behave similarly.

I initially ran the SCM 35’s using an Audio Research SP6B preamp driving a Conrad-Johnson MV75-A1. The C-J amp was not able to control the woofers well enough for my liking, and I brought a heavily rebuilt GAS Ampzilla out of retirement, which did a significantly better job of driving the speakers dynamically.

After replacing my preamp with a Cary SLP05 with ultimate upgrade and my Ampzilla with a new production SST Son of Ampzilla II amp, I upgraded my speakers. This combination drives this speaker really well and is what I would recommend. I have also driven the system with Schiit Freya+ and original Saga preamps, and would think amps like the Schiit Tyr or PS Audio BHK series would be great.

 have also heard the active ATC SCM40’s numerous times and to my ear they have a sense of tremendous dynamic control and really excellent imaging and sense of inner air and space, as Lone Mountain (the US distributor) suggests. I think my setup approaches the active ones, just a bit less effectively. I wholeheartedly recommend both setups, but definitely with solid state amplification and tube pre amplification.

There isn’t a single dealer in the northeast that has ATC SCM50 or higher.
I’ve auditioned most other brands ranging from 10-300k, none of them had the dynamics, timbre and accuracy of ATC except Avalon acoustics Saga. Those are the very best I’ve heard but the price is out of reach. 

I auditioned SCM40 active and passive. The active demo was controlled by ATC preamp. The passive was driven by North Moon 641/681 DAC ( this DAC was also the source for the active setup)

I preferred the passive setup; I ended up purchasing the passive SCM40 v2 to use with moon 761/791. Super happy. These speakers have taught me a lot regarding natural sound and high performance. I understand why there is so much praise for ATC.