The V4B is great by itself.
I am first of all acquiring a Noble so that I have a great DI when I don't use my amp which is pretty heavy and inconvenient to move around. Or when I want to record or play at night without my V4B on.
However, I ll sit at home with a Noble and a V4B. I am wondering how it will sound when plugged in the effect return to bypass the V4B preamp.
The V4B has 4 x 6L6GCs in the power tube section. You're going to get tube sag - basically the same kind that you (IMHO) get from the tube on the DI.
If you plan on using the line out, which doesn't have a tube on that out put, you'll get some nice compression.
The only question for me would be to compare the frequencies that the Noble has vs. the Ampeg. If they are close to being the same, then you probably won't hear a difference.
Again, I am no expert, but this is the way I see it.