I decided to try a subwoofer with my two channel system and took some measurements of the frequency response of my Tyler Linbrook Monitors and Polk Micropro 1000 using the realtraps test CD http://www.realtraps.com/test-cd.htm. After I got the sub dialed in as far as matching phase with the Tylers and finding the optimal room placement, and crossover, I think i got it to blend quite well with the exception of one dip in frequency response.
I ran the test first with just the Tylers and then with the Tylers and the Micropro 1000. I used the line level inputs with the Micropro crossover set at 60 Hz. Initalliy I was a little disappointed at the SPL levels I was able to get from the Micropro using my Musical Fidelity A1 but I can see from the chart and from listening that I am in fact getting substantial support from the Micropro in the lower frequencies - plus 10 db in some areas. In the chart, frequency is shown on the x axis and SPL level on the y axis. Interesting cancellation dip around 50 -55 Hz.