BadDNA - FYI AVA's ABX comparator, now back in stock after 2 years, has a built in (approximately) 100hz selectable filter for the mains that can be set in or out of circuit with a touch of the remote. It also has a sub output, which is full range so your MiniDSP would do the crossover, that is volume adjusted for the A B comparisons that the unit is designed for.
My system sounds like it might be similar to yours.
Most people I've encountered, including myself, feel they like the mains running full range better than having the low end filtered out once they can hear the two options switched instantly at the users discretion.
However, the ABX is RCA only (no XLR) which might be an issue if you are thinking about the new digital preamp.
Dan