I actually had a similar problem when I lost 40 lbs., recently, but I've gained back 20 lbs., so the point is now moot. ;)
Seriously, I was frustrated that I lost too much muscle mass at the same time my body fat percentage was dropping, so I decided to focus on building lean mass. I'm eating a lot more lean protein, now, which seems to help. For the time being, while I build up my core strength, I'm OK with having a belly. That said, I'm also doing a LOT more ab work, which seems to help.
Regarding the butt, I actually just met with a trainer, yesterday, to help me come up with a better routine for butt, shoulders and outer biceps. For glutes, he stressed the need to do bench step-ups, lunges and squats to increase glute size. He recommended doing this "big muscle group" one day, and then 48 hours later, doing another more traditional "small muscle group" leg workout (extensions, curls, presses, etc.).
Regarding stance, he did say that it was important, but he said that the angle is actually the most important factor. He said you shouldn't go too much beyond 90 degrees or the quads tend to take on more of the load, thus reducing the glute benefit.
Best of luck. I'd love to compare notes in a month or two.