"Thanks! Parts express folks seem to love those Titanic subs.Did you recommend the 18 incher due to more cone area?"
the 460 will probably sound a hair better on the top end with its higher sensitivity and better inductance control. it will also have higher low end spl because it can move more total air.
"I dig the amp and its EQ. Would it be safe to run 1 sub in bridged 4 ohm mono? Is that too much power?"
it should be fine, so long as you don't get stupid running 10hz sine waves through it in free air.
edit: you can also limit the voltage in the dsp in order to prevent putting too much power through your driver.
one per channel is very safe power, almost underpowered. i was just suggesting a conservative approach.
"Alternatively, is it safe to run that amp in 4 ohm stereo with the second channel unoccupied (only 1 sub)."
yes it is safe. our preference for symmetry causes lots of folks to ask this question, but this is one place where our intuition fails us.
"I apologize if these are obvious noob questions."
no worries. questions are questions. the foolish thing would be not to ask.