Just to followup - earlier today I replaced the factory DCC decoder in that Atlas GP38-2 with an ESU 58751 micro DCC Direct ‘Atlas’ Legacy unit. All in all, it was a walk in the park - super easy, and the loco now operates beautifully throughout its entire speed range. Best of all is that nice little crawl at speed step 1, with cv2 set at an entirely reasonable value of only 2 (recall that the piece-of-crap NCE factory install, even with cv2 = 60, wouldn’t get the loco to budge until SS 8 or 9!).
I carefully “milled” down the rear platform as demonstrated in a few TRW sound installation tutorials so as to accommodate the speaker (just a couple minutes work on my 1” belt sander did a wonderful job - nice and even, smooth & shiny!). To this end, an ESU 9x16 speaker should be arriving in another day or two.
In short, a very satisfactory outcome, despite a frustrating and disappointing “out-of-the-box” start.