I generally advise against using bass for home-brew development. Bass is not a very well thought out assembler. I've never used it for 65816, but for other architectures like MIPS has some serious design issues that cause invalid code to be silently accepted by default, which is normally a disaster when it gets to debugging.

For MIPS at least, one of the completely wrong design decision has been to map basic register indices to raw numbers. For instance, in bass, this is a valid instruction:

    add 2, 4, 5

which means "add register 4 and 5 together and write the result in register 2". Normally, one would write that line with the register aliases:

    add v0, a0, a1

The problem comes with the fact that MIPS has also a "addi" (add immediate instruction) that you would use like this:

    addi v0, a0, 5

"add the immediate value 5 to a0 and store the result in v0". So I guess the problem is clear: what happens if you instead write:

    add v0, a0, 5

There are two possible reasonable outcomes: either the assembler should reject the above line as invalid, or it should silently convert it to "addi" which is what GNU as does for instance. Instead, with bass, the above is a perfectly valid line which gets assembled to the same of "add v0, a0, a1", which basically silently generates wrong code.

I think bass was a quick hack that overgrew its intended goal. I suggest to use something more mature.