The Speakerpower SP4000 is great, and is available to individuals. It is only the Speakerpower DSP units that require a large purchase.
If you actually measure the musical SPL output of subs driven by the different amps you have, you will probably find they are different animals, even if their output ratings are the same.
http://www.diyaudio.com/forums/subwoofers/187472-speakerpower-torpedo-sp1-4000-plate-amp-review.html
Ah, yes...didn't read the DSP qualification close enough....
My bet is the 4050 is the pick of the litter....