Here are two options to try depending on what you hear, as the frequency response from the near location will affect which sounds better.
First try and run them as you did before, with the gain set the same, and let the 8033 do it's thing and then set the distance at 6-15', listening for what sounds best (assuming you can't measure). Listen to some good sounding drums or other percussive bass sounds. The reality is that at these low frequencies, the correct acoustic distance (it what that of the sound arrival not just the physical distance) will vary with the response in the room and of each subwoofer. This is where some of the auto calibration systems can be of help. If you are having issues with localization of the near subwoofer, err toward a lower distance setting, but do confirm it is better with actual listening.
If the near subwoofer sounds too prominent or calls too much attention to itself, I'd then suggest switching to the L or R input rather than the LFE on the two MFW-15s. You will need to increase the level after doing this. Then set the low pass to its maximum of 180Hz on the front subwoofer. Now set the near subwoofer to 60-100Hz. This rolls off the highest frequencies a little, which are more localizable, but more importantly it does add 1-2' of signal delay to the closer subwoofer. If this helps slightly, but not enough, you can then try adjusting the phase knob on the near subwoofer as increasing the phase from zero also adds group delay which can help the cause.
In some cases you can get great results by just running them the same, but sometimes there are other artifacts such as localization issues, or frequency response issues. This does become much easier if you can actually see the frequency response of the system, but even though there are some time related measurements which will indicate likely directional ques, they are much easier to decipher at high frequencies than low, where a quick listen is the best confirmation that other contributing factors haven't been overlooked.