I've only used double flute straight bits for cutting circles and mortises for flush mounting. But I've heard that a spiral upcut works pretty well too. I'd do 3 passes for 3/4", 5 passes for double layer baffles. You can probably get by with 2 pases but I've snapped a few too many straight cut bits doing that.
For general speaker cabinet work, I'd recommend a straight cut (upcut or flute), flush trim (a straight cut with a ballbearing), a roundover (1/2"-3/4" for rounding corners, ports, braces, etc.), and if you are building home audio speakers then a mortise for flush mouting.