Down firing vs. normal subwoofer

None. Down firing runs less risk of getting damaged by something...vacuum cleaner, knocked over chair etc.

As is always said you can't localize (identify where it coming from) below 150Hrz this is not always the case.To wit many folks prefer two subs though theoretically you should only need one.But I think the type of sub,it's components,placement in room should make bigger difference than if down or front firring myself like a large single driver for HT but for music prefer speed,finesse ,and tonality (accuracy) or multi driver subs by Vandersteen (excellent value if only down to high 30's),REL,or Martin Logan all of which use three small drivers to equal surface area of large single sub but because they are small drivers they are faster in response and more pitch accurate.The ML uses front firring the REL and Vandy use down firring speakers.So I don't think i matters other factors do.
Chazz

"As is always said you can't localize (identify where it coming from) below 150Hrz this is not always the case.To wit many folks prefer two subs though theoretically you should only need one."

That's for minimizing bass nodes, not localization issues.

Chazz,

I think those who argue for multiple subs do so with the idea of compensating for spectral peaks rather than localizing source.

db