** Man of Leisure **
No, that is not correct. First of all, I want to make sure you aren't talking about the LPF for LFE setting. You can't output a test signal to the LFE channel using Java.So that is why you set the crossover on your L/R channels to max 250hz to push those hz to the subs, and disconnect the L/R speakers. I noticed the channel options right off the bat using ASIO.
Second, if you want to test the performance of the subs by themselves, you want to keep the regular crossover engaged, since that is the way the subs will sound in normal use. Output the test signal to either the left or right channel and simply unplug the speaker cable to isolate the sub signal.