I own a SC Reference but would recommend the two Ultras
The SC Reference is the only Def-Tech sub I would recommend. It is powerful and can be downright scary but I honestly do not think it is in the same league with the PC Ultras. The SC Reference can make the floors tremble and is really a great sub in a relatively small (and heavy) package, but I do not think it to be in the same league as the PC Ultra. I have not heard the PC Ultra and I can tell you two SC References would by all means not be bad at all, but the PC Ultras would most likely have a little more headroom and dig a touch lower, although I can get about 108 db out of my Reference @ 17 Hz in room which is respectable, and I am sure has some room gain helping it.
I am no fan of the SC 1, 2, or 3, as they do not perform to the specs Def-Tech lists in the same +/- 3 db spec most other Manufacturers use, but the Reference is in another league all together from the other Def-Tech subs.
I hope I helped you out, if it was up to me I would go with the SVS's based on customer reviews and their outstanding reputation.
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