When I clean my cages I use a product called "Pet Focus" which was designed for cleaning around birds. If something is safe to use for birds, it's safe to use on just about anything, as birds are ridiculously sensitive to fumes. It has a faint clean smell that eventually disappears and you're left with no odor whatsoever. It gets anything off. I swear by it. It's also supposed to be very effective at killing many types of bacteria that some other cleaners wont get. If you have a bird store near you, or can find it online, you might give that a go.

Also, as for the aspen, I find it to make the smell worse, not better. It holds all moisture and just allows everything to sit in the heat, thus producing that funky smell. Just my