McCormack SMC Audio upgrades: how good are they?

Chiho
You state your budget is $3K-$4K for the amp, is that after you sell the DNA? If not find a used amp here on Agon, if you are not happy with the way it compares to the DNA 125 Gold you can sell it and try another. The point is you won't loose your McCormack and you could still have it upgraded to Platinum at SMc Audio.
As another option find an inexpensive DNA-125 and have it upgraded to Gold and the two monoblocked. Not sure if this thought had crossed your mind. Of course you would need to search for a balanced preamp after that.
As for the Preamp I would suggest a used McCormack and have it upgraded as well. I have my TLC-1 scheduled for the Ultra Version at the end of this month.
Have you talked to Steve to see if his upgrades will make the improvement you are looking for?
The point is I would hate to see you sell your DNA-125 Gold and wind up wishing you had it back and looking for another for sale. Like you said they don't come up here often.
By the way did I mention that I'm a fan of the McCormack gear.

I had this same thump with a pair of DNA-1 Golds that were connected in a vertical bi-amp configuration to my Vandersteen 3A Sig's. I called Steve to discuss this and he said there shouldn't be any problems. Four years later I had a pair of 0.5's upgraded to Platinum mono-blocks and the Gold's went on their marry way. Still have the Vandy's and yet to have any issue's. I have to admit I was a little concerned every time I turned them on but that subsided with time.
Hope this helps with you situation.