Re: FIR filters
Sorry Mark. I'm was using the terms a little loosely.
Just to clarify, I meant running an IIR filter in both directions gives the same magnitude response as having run it twice in one direction, but the filter's effective phase response is linear. It's like using a FIR filter with symmetric coefficients from -inf to +inf time but with the computational efficiency of two IIR's. Clearly not possible realtime. 
Regarding frequency domain processing... FIR filters can be implemented directly in the time domain, which is computationally expensive, or using transforms (DFT/FFT's). The two methods are, on most processors, essentially numerically identical. Textbook transform based filtering has block latency which is a function of the transform size. Check out
https://en.wikipedia.org/wiki/Overlap–save_method
https://en.wikipedia.org/wiki/Overlap–add_method
For realtime processing of anything longer than short filters, the latency often isn't tolerable. With the Huron workstation, Lake DSP developed algorithms for very efficient, very low latency filtering using very long FIR filters.
Regarding the Lake Processor "Alignment Delay" setting, this setting locks in the bulk delay of the linear phase FIR crossover filter. It's a convenience so that the whole system doesn't have to be realigned when the crossover frequency is changed. ("Alignment Delay" only applies to linear phase FIR crossovers and is ignored for "classic" IIR crossover modules.) As the crossover frequency is varied, different FIR filters coefficients are used. The alignment delay sets the upper limit on the length of the filter. For example an alignment delay of 5ms means the linear phase FIR crossover filter can't be longer than 10ms (due to filter coefficient symmetry) or ~960 samples @ 96 kHz. This limit on the filter length then limits how sharp/steep the filter can be at low frequencies. The Lake manual has tables showing how low in frequency the different linear phase filters can go.
