Bryston ST vs NRB low end!?

Alright, so I have two brystons sitting here: one is an NRB and the other an ST.
Now, I'm planning to keep one of them for monitoring mixes in my studio through a cranesong Solaris.
Although they characteristicly sound very much the same, the NRB just has a lot more low end, it just makes the low end stick out on my Proac SM 100.
The ST seems to be a lot more balanced and the low end is more laid back embedded in the sound while with the NRB it sticks out. But somehow I like it when the NRB has this protruding Low end.
Like what's happening? Is it more dynamic than the ST while the ST is more compressed or is it an intentional EQ boost for "HiFi" sound while the ST is Actually flat?
I don't know if that low end extension would help mixing songs or hinder it.
Can you explain to me what's happening between the two amps?
Well if its more dynamic you would be hearing that across the range, right? And if its boosted then you would be able to EQ it right back out again, you being in a studio and all, right? But instead of any of those observations the one you make is that you like it.

So what is the problem? That you like it but you can't explain why? I like chocolate ice cream, haven't the foggiest idea why, chocolate after all is bitter, but I like it. Can you please explain why so I can be okay with liking chocolate?

Seems to me since you are using this for mixing what you really want to know is which will make your mixes sound the best. For that you  simply make a mix with each amp, play it on a couple other systems, see which one you like the best. This after all the goal, right? To produce good mixes?

Do you think it's a defect that my ST sounds leaner on the low end than the NRB?
The thing is that I had already sent in the ST for servicing, they swapped some switches and transistors on the right Channel but it still Sounds the same.

The both amps are set to equal volume, it just sounds Like the lows in the NRB are dynamically protruding more and kind of kicking out more low end out or my speakers.
Is there an explanation for that?
To me, Bryston amps have what I call "recessed" bass. But I've not auditioned or owned any models earlier than the ST series.

Sounds to me that Bryston implemented some technical change between the NRB series and the later models lines that produced the difference you are hearing.