Hello khiak,
What are the dimensions of your room (length x width x ceiling height)? What are the brands and model numbers of your subs?
You can definitely achieve very good bass response at a single dedicated listening seat with 2 subs. Using the 'room crawl method', described to avanti1960 earlier, in a sequential manner (first sub#1 and then sub#2) is probably the best way to position them.
In my experience, 2 subs will provide much better bass performance than 1 but in my opinion 4 subs will provide the best bass performance since line array sub configurations are not well suited for small rooms like distributed array sub configurations are.
The 4-sub DBA system concept works incredibly well in virtually any room and actually is perceived as making a small room seem like a larger room.
The key to a DBA system being such an exceptionally high quality bass solution is the use of 4 subs; the brand/model, size, power, price and type (self amplified or passive) are much less important than the fact that there are 4 of them active in the room. There are also no requirements that all subs are identical and subs of varying driver sizes, power, prices, types and bass extension are viable.
If you have a smaller room, however, I'm curious why you chose to add 2 subs with large 21" drivers when adding 2 more subs with 12" drivers is more logical and physically suitable?
Tim
