[Brian Cheney]

There are two basic ways to achieve a low cutoff frequency in an enclosure: high moving mass/low compliance of suspension, and low moving mass/high compliance of suspension.  Traditionally VMPS has fallen into the latter catefory.  TC Sounds drivers and PR's fall into the former category. 

Larger enclosures work best going the low mass/high compliance route, smaller ones favor high mass/low compliance
configurations.  You can mix the two within limits.  However, a Megawoofer (low moving mass, high compliance in a big box) usually won't push a TC PR *mass loaded to its limit) with sufficient force.  The PR begins to resemble a solid baffle wall rather than a a compliant, pistonically moving diaphragm, and system F3 jumps higher, which is not what you want.

I have oversimplified greatly here, but that's the answer to your question.

[Vr3]

So wouldnt two TC Sound drivers capable of moving 3" peak to peak in a box 5.44 cu ft which is basically the recommended size of the 12 and the 15 combined a good match?

I would think those two drivers could move a TC Sounds PR without issue...

[Vr3]

Kyle on the TC Sounds forum has even suggessted a 5 to 6 liter box with two 15" PR with just one active 15" for nice low tuning...

The Larger enclosure specs out around 5.4 cu ft - so I cant see why a 12 and a 15 occupying the same space cant adequately move -one- PR

[Vr3]

I am total n00b, but since I do not know how to model this correctly, I modeled the 12" with the 12" pr and the 15" with the 15" pr - the 12 in a 2 cu ft box and the 15 in a 4.5 cu ft box

Both PRs in the models were at full mass -- the 12" had usable output to 20 (-7db) - little bit of eqing and it would be golden, the 15" had a -6db at 20hz, -10db at 16hz

I imagine with that slot port on the bottom of the Larger it very well could enhance that low end further?

[Brian Cheney]

Yes, a 12" and a 15" TC active would push a 15" TC PR, suitably mass loaded, to a low frequency and achieve an F3 of 20Hz with a little EQ.  However, you've got a lot of expense there, over $1k in parts alone plus an amp with parametric EQ built in.

A single 15" TC active with very long dual 4" flared port tubes will have an F3 of 20Hz in the Larger enclosure without EQ.  Tune the box to your taste by adding enclosure damping with BH5/SR71 and/or fiberglass sheets and you're done.  Max output levels are about 3dB lower than with combined 12" and 15" actives, but what the hey, most people won't need them even in an HT application.