what pre amp for ATC SCM 40v2 spkrs + 250.5 Pass Labs pwr amp

I see thank you for the further insight. 
So then I kindly ask which SS pre-amps are you aware that meet these ideals that you mentioned earlier? 

Many thanks for the responses and  @atmasphere,

I'll need to further research to digest the ideas you have expressed. It does sound like I'll have a hard time finding an ideal since manufacturers don't market with these concepts, let less explain them. 

So basically I need to:
1) check the output impedance vs frequency graph to be flat
2) relative to whatever load (value) used to measure. 
3) make sure it is AES 48 standardized 

Sorry the last paragraph:
"To this end, the output of the preamp can't reference ground- instead the complete signal is set up so that the non-inverting output references its opposite and vice versa; ground is ignored and used only for shielding. Traditionally this was done with an output transformer when tubes were king and still is with solid state because of the grounding issue. Regardless there are tube preamps that support the standard. "

Means that old tube amps are more likely to be what I'm looking for,
and solid state has always been more common of what I'm looking for?

I think to sum it up, these specs that you speak of seems like the ideal pre-amp "in general", as oppose to the "best" pre amp for my scenario?
I mean I'm assume everyone wants what I want of course lol
 (wider soundstage, more bass, more detail)....?

Now the simplest solution, I suspect is just to grab a Pass Labs pre-amp from the same era...

Thanks for your input @three_easy_payments, 
which model @tomic601 ? and what kind of system are you running?

@atmasphere  

I've done some research and my findings are rather interesting. 

A) I was aware pro gear is all balanced- as they need to run long cables etc. As such my crown amp only has XLR inputs but they are also NOT AES 48 certified, there is no certification on the amp nor in the owner's manual, nor is it on newer crown amps as shown on their website. 

B) Secondly, I think I've only really seen (in 20years) 1 amp or pre-amp (I don't remember which) that only had XLR connections. 

C) PIN 1 in XLR is ground, if source is RCA then PIN 1 needs to be jumped with PIN 2 as PIN2 then becomes ground (and not balanced anymore). This case is true for my PASS amp 250.5.
Meaning that in differential configuration on my PASS amp PIN 2 and PIN 3 are not connected to ground because PIN 1 is ground.

I also don't understand why you would promote AES48 as a standard that allows for "superior" sound when "almost" no one uses this standard. Pro audio and Audiophile gear has been around for over 40years... 

You also mention IIRC Rowland made a balanced amp but didn't "support the standard" so this means that manufacturers don't need to support the standard in order to make a true balanced design? 

So this basically means there wasn't a problem in the first place, AES 48 is just a stamp of approval (that most manufacturers though following the design chooses not to apply for the certification). And as such brings me to the next point where you say

  "But you will find that you have to be more careful about the interconnect (which might cost as much as the preamp itself) and you could also run into ground loop issues."
Why would one need to spend $1000 on RCA interconnects to avoid a ground loop? when one could just use the XLR and it's shielded.

I'd like to get your input in all this confusion. 
Thanks