I used the 'Ask Dr. HSU' option on the hsuresearch.com site. I described my media room, my Axiom speakers and other HT equipment, and added the following comment:

Dr. Hsu: which sub should I choose? I want a good clean extension to 20Hz with a flat response between 20 - 150 Hz? I know that the VTF3 MK2 is on sale. However, the VTF3 MK3 seems like an attractive choice. I don\'t want to pay for more than I will need, so I will appreciate your input. Thank you!

He replied: I would certainly go for the VTF-3 MK3. The extra headroom in the low bass is definitely worth the extra money

I'm going with the HSU VTF 3 MK III (no HO).