Upgrade Quandary: Ref 75SE vs Ref 150 (non SE)

@jo1mtb Since you are looking for "more definition and control, without messing with the rest of the sound" have you considered isolation platforms for your speakers? I looked at your system pic and if it is current, it looks like you are using some type of footers (stock?).

Something like the Star Sound Technologies Sistrum platforms. I personally have not used them, but they have consistently received strong endorsements from owners (that I know) that have considerable experience with speaker performance.

It is a different direction and, likely, less of an outlay. I think Star Sound Technologies offers a full return guarantee, so it’s an easy test to conduct. If you really like the results, any subsequent amp upgrade will just add to what you’ve gained by using the platforms.

Hope that helps. Best of luck.

ARC 150 non SE and then, when the tubes are used up, replace them with KT 150s. I have gone through the Ref 110 with 6550s, Ref 110 with KT 120s, Ref 150 with KT 120s, Ref 150 with KT 150s--all with my Sashas. I have also heard the Sashas with The Ref75, 75SE, Ref 250s and Ref 250SEs.
 IMHO, the Ref 75 and 75SE will not have the sheer grunt and force of the non SE Ref 150. The added bonus is you can get to at least 85 percent of Ref 150SE just by installing the KT150s at the appropriate time. The standard Ref 150 drives the Sashas almost as well as the Ref 250s and much better than the Ref 75SE.
 Ken Kessler from HIFI News thought he had gotten everything he could out of his Alexias with the Ref 75. Then he heard the Ref 150SE and said it simply showed him things he never knew existed and that the Ref150SE was "supernatural" and "a level beyond". It replaced the Ref 75 as his reference. The difference between the Ref 75 and Ref 150 is even more pronounced with the Alexias than it is with Sashas---but it's still a huge difference with the Sashas. 

 I totally agree with gpgr4blu. You could always the Ref 150 upgraded by ARC to the SE version later if you felt the need.

The difference between the Ref 3 and Ref 5 is not that big. The difference between the Ref 5 and Ref 5se is much bigger. The difference between the Ref 3 and Ref 5se is substantial. The Ref 5se is just magical with a huge holographic soundstage. If close my eyes and I would swear there is a piano in my living room.

 I also agree that isolation is very important. When I went from a Solidsteel rack to a maple rack I could hear an audible difference. Maple sounds smoother, less mechanical at least with tube equipment. I noticed the biggest improvement with my amp. I now have a maple Zoethecus rack with the Z-slabs. Each of the shelves is layered with RF shielding. I have each piece sitting on large stainless steel cones.  The other thing that made a big difference was replacing my power cords with Shunyata Alpha HC cords. They just enhanced all the great qualities of my Ref 5se.

I would not buy a non-SE ARC amp - the sonic gains are too much. I would save up for a Ref150SE or get a Ref 75SE in the interim and see how it performs.