You are putting way too much weight on DF. Don't worry about it. Any amp worth owning will have adequate DF. This isn't one of those figures where higher is inherently better. If the amp has a DF above about 20, then it has adequate damping and going higher won't yield any audible benefit.
Likewise you are worrying too much about THD. There's no audible difference between 1% and .00001%. And given the way the THD spec is given for those Soundstreams, I wouldn't be surprised if the THD figure is more of a "minimum" figure rated at a low wattage and the RMS power rated at the more typical 1% THD figure. But, even beyond that.....assuming they did rate power output at .02% it's still possible to obtain well above .02% THD from the amplifier.... it all depends on how you were to set it up. About any amplifier worth owning will be capable of 1% or less THD....yielding no audible difference in distortion.
If you want the Soundstream then there's no problem with that....just sounds like you are choosing them for the wrong reasons. Given they are 2-channel amps I would presume them to be class A/B.....which would be a heck of a lot of current draw on your system compared to a class D of the same power.
