Yes, you can add another subwoofer brand without a problem, in fact you should try another subwoofer brand since the Deftech subs are very performance compromised for the sake of size. What you want to do is use the Deftech tower subs along with another sub. Get two of
these, use them at the subwoofer outputs on your AVR. Use both tower subs and a real subwoofer. I would be looking at Hsu, Outlaw Audio, RBH, SVS, or Rythmik subwoofer brands. One thing I would do is use a high subwoofer crossover point in the AVR, maybe 120 Hz to 160 Hz. Send that range to the tower speakers. Use the regular subwoofer's low-pass filter at 80 Hz, so it isn't trying to play back bass that high. Make sure you use room correction like Audyssey or YPAO or MCACC.