Two cheaper subs with lower excursion shouldn't be any different than a single long throw unless it's a 15". It's more expensive to make a respectable long throw sub. Although, sub installs that are cleaner typically are perceived as lower output due to lower distortion.
The biggest issue with output is location/placement since it makes harder to match with mid woofers. Btw, Sundown uses a Korean build house for their amps and they do a great job. Customer service is top notch. I wouldn't shrug them off. Their 8's do very well with small enclosures too. I think it'll be plenty for you. I'm pretty sure they have setups that reach into 140's with just two 8's.