I have already tried Creamback 65 on the top with V30 bottom. Creamback has somewhat muffled top end and sound quite flat. H75 has boosted lows and highs, maybe too much. It sound more piercing than V30. Redback is quite flat but not so much as K100, sound like relaxed H75.
Originally Fatbottom was 2525 version (4xgreenback) it sounds massive, but I'm one of poeple who hear GB piercing when pushed, much more than V30. Of course I tried V30/GB mix, but You still hear more GB than V30. Interesting thing is that I had early Fatbottom like 10 years before and it was 2530 version (2xGB top, 2x Marshall V30 bottom) but that cab has extra panel inside, in horizontal direction (like two 2x12 glued together ) and it has really some magic included

But this newer one never sound the same. Still one of the tightest cab around.
I was closest to solve this issue when I put Scumback's in. They are really greenback-ish, more than newer GB's but has more pleasing, fat and rounded tone. I tried 65watters H75 and J75. But in Europe is difficult get Scumback for reasonable money so I keep 'em for 2x12 rearloaded cab.