I use DS<() a lot in Supersonic Ball, because the animation speed for sprites/tiles varies from a level to another.

≥≥0 doesn't really require any code at all in ASM. Every other test really does what you did above: it subtracts the value to test for, and see how the answer compares to zero.

While we're on the topic of optimizations... Would something like A-128≥≥0 be more optimized than A≥≥128? And if so, why is the ≥≥CONST entry in Auto Opts.txt listed as taking a whole 11 bytes more than ≥≥0?

A-128≥≥0 will fail for 128 values close to -32768. That's because after you subtract 128, they become positive and then the comparison will think it is greater. The ≥≥CONST routine takes this into account.

I'm kind of new to programming, and there are a few things I'm unfamiliar with.
1. What is a nibble?
2. What can you do with nibbles?
3. In what kind of situation would nibbles be useful?
4. How are nibbles used?

So you could theoretically halve the space a variable takes?
4. How are nibbles used? commands list, right?

Its mostly electrical terms.  Like male and female connectors, wire strippers, tickler coil, screws, nuts, etc.

You don't "halve" a variable unless you can actually do it.  If the variable has to hold more than 16 possible unique values then a nibble is not sufficiently large enough to hold your data so you would have to use a byte or word instead.  Some tile mappers for instance might always use 16 or fewer tiles per room and so you can load a palette for each group of rooms and then compress the maps to half the size.

Another example is if your character is always aligned to an 8x8 grid, then the X,Y coordinates can be written to a single byte rather than a separate byte for each variable since there are only 12 X possibilities and 8 Y possibilities.  Keep in mind though, that you should only do this if memory is a problem.  Accessing bytes, and sometimes words, is actually fastest and smallest in the program when it comes to reading memory even though the actual data is bigger.

FOOD ... j/k

Btw, here's an example where nibbles are really useful.