If you can find cheap jbl 15" then you might move more air with them (liter/$).

I have a IB subwoofer using eight JBL GTO1514 and I have eight SI HT18 on the way.

In Denmark one JBL GTO1514 will cost me about 1000kr (danish kroner aka danish money) when I bay in bulk.

One SI HT18 is 1850kr (when doing Google to conversion use DKK for danish kroner)

Prices including shipping and tax.

One JBL GTO1514 will move 1,2liter air one way (for a total off 9,6 liter when using eight) and one SI HT18 will move 2,67liter air one way (for a total off 21,36liter one way when using eight).

JBL GTO1514 cost 833kr (127usd) per liter moving air (one way).

SI HT18 cost 693kr (105,7usd) per liter moving air (one way).

So HT18 saves me 140kr (21,35usd) per liter air

But that is in Denmark, you may be able to buy cheaper subwoofer speakers then what I can in Denmark.

I used a reshipping service, and that cut shipping to half. Now they (HT18´s) are on a boat towards Denmark.

**S**o the way I see it, it all comes down to how much it will cost you in shipping and tax for one SI HT18.

(The HT18 will go in big sealed enclosure, and JBL GTO1514 will be sold off. I be moving my cinema room and the new place will not hold a IB subwoofer)