So there are a couple of things I'm noticing, and plenty of options to go with when refining:
- You probably want to stick with just B/W, no greys. You don't need them, and your forms will look more definite if you stick to B/W only. It may do to have the SB drop shadow in black and it's just not visible against the circle, or you could turn it black and have it outlined in white when it's against the circle.
- You'll do better to stick with one line weight, also. If you fill in the banner lines, instead of stroking them, then those lines approach the stroke weight of the SB outline and the circle outline. This is where details start to tell us things about the objects; meaning that if the line weights in SB and the banner are the same, but the single line weight of the circle's outline is thicker, then you've subtly grouped those three objects into teams. One team is the important legible stuff, and the other team is the bold background object. They're already teamed by being predominantly black in the background and predominantly white in the foreground, but those disorganized stroke weights aren't helping. So, see if you can get it down to two stroke weights for the whole thing, and use a single stroke for the circle outline instead of a double.
- On the banner, there's no reason to have the top line terminate at the right edge of the circle, but have the bottom line terminate past the edge of the circle. Everything else about that object is symmetric and it unbalances a mostly balanced thing to do that. You may have done it because the tail of the S needed an anchor, but...
- The over-the-edge SB sizing isn't working so well because the balance of what's on and off the circle is wonky. See how the tail of the S seems to stick out so far? That'd be fine if the B hung off more, but I don't think you want that. Think of the white in those letters as volumes, and you want the same volume of white hanging off in each letter. You may have to size them differently to pull them off because of the next (related) point.
- See how the various volumes of black space around the letters S + B are also quite awkward? They're awkward because the overall volume is very lopsided (most of the black is contained in that one space between the S+

and so some of those spaces call attention to themselves. You'll want to kern to get the volume of black between them to more closely match the volume inside of the B for sure. The other volumes may even out a bit when you do this, but it may also involve subtle sizing readjustments of your major players.
Basically, you have your overall layout pretty close, and now you're making noodly little adjustments in an attempt to perfectly balance things. You may still have to make major concessions in getting it all to work, but it's pretty close.