If you look at the DCS205 FR graph in the "specifications" link on the PE site, you will see its upper freq response. It looks to me that it may cross as high as1 KHz (e.g., ROT is 1/2 the freq. of the woofer's breakup at 2 KHz.)
To design a proper speaker, you need to put the FR graphs (in numerical form .frd, .zma files) into modeling s/w like WinPCD. WinPCD will also model the "box", low end freq. response as well as baffle step response. And it will use the upper freq. of the graph to mate to a mid (or tweeter) by allowing XO values to be entered.
Pick two of: (a) Low end extension; (b) Efficiency; and (c) Size. You can't have all thee - Hoffman's Iron law (the H, in KLH).