I know mixing fronts and surrounds is not (in general ) a good idea, but what about subwoofers. I just received my HSU VTF3 MKII amp back from repair and placed the sub in my new HT area (after the sub crawl) and in this new (very large space) I'm just not getting the punch I used to get in my smaller HT area.
I would love to find another VTF3 MKII to add to it but they dont make them any longer . I have found some other used subs that I would consider but they are not HSU.
Opinions please.

I've had a similar situation with my SVS sub. I want to add a second sub, but don't want another cylinder shaped sub. I was told that it would be best to try to match the specs of the other sub. I do know, however, that a number of people here that have multiple subs have different models at least, if not different brands.

I think that as long as you have 2 (or more) quality subs, you should be ok with a little tuning.

I think the main issue with differing subs if they perform differently at the higher (or lower) frequency ranges. For example if you have smaller speakers with a X over at 120Hz, and 2 subs but one of them can't play well up to that 100-120Hz region, you'll have a relative dropout in that region. Differences in output should be able to be compensated by the gain level on the sub itself. I have no proof of this, it's just my logic at work here... I assume someone actually knowledgeable on the subject will chime in soon.

Really, much ado about nothing. I have FOUR subs(three different manufacturers, one down-firing, three front-firing) and as long as they have somewhat similarly capable frequency ranges(especially at the bottom end) and power, volume balanced and everything properly in phase, it will be no problem. Obviously, as we all from time to time may add or change equipment in our systems, a model we may have and like will no longer be available so when we see something we do like with similar specs and even price BUT from another manufacturer, it is quite common for this to occur. If you want something close to the HSU model you have , check the "Outlaw Audio" website, the LFM-1 Plus and EX are essentially slightly re-badged HSU models with similarly good performance but a little less costly.

Practically speaking, moving subs around to different locations in your room, even with the same manufacturer, will have a considerably greater affect than using subs from different manufacturers. Go for it.

Brady, I'd be inclined to agree with Casey. The question shouldn't be whether you can get perfectly matching units, but whether you can get a new one that's good enough to make a net improvement. The answer would be yes as long as it's of reasonably similar capabilities.

Thanks guys. That's kinda what I was thinking but just wated to run it past ya'll.

I think casey nailed it.

The other thing that came to mind is...

The Behringer Feedback Destroyer (BFD) is fairly cost-effective. Combined with the Room EQ Wizard (REW) software, you could have a powerful (if somewhat tedious) method to equalize each sub independently. Poor man's SMS-1. Check out Home Theater Shack forum for all the information.

But, honestly, I think that's overkill.