This is a highly debated subject. You are correct, most subs are detectable even at low frequency. Hearing is not the only thing at play here. First off I would suggest lowering your cut off frequency down to 30Hz since your Studios are 3db down at 33Hz. Localization is very easy on a sub with a 60hz cut off anyway. Set at 60hz you probably have too much bass in the 30-60hz range.
You can try the stero sub set-up i.e. one near each speaker. Effectivly, you are creating fuller range stereo speakers with the two subs. The drawback can be cancellation of certain frequency's due to the sub spacing. Think about it like dropping two stones in the water some distance apart. the waves colide and some cancel while others add. The stereo effect is difficult to distiguish as the frequency goes down, so low frequency tends to sound mono. Stereo seperation becomes less important and sound wave cancellation can become an issue. Stacking subs is like throwing one bigger stone in the water. There is probably not much stereo information below 30hz so you may find it gives you better impact. You will need to experiment with your room and set-up to see what works best for you.
You can try the stero sub set-up i.e. one near each speaker. Effectivly, you are creating fuller range stereo speakers with the two subs. The drawback can be cancellation of certain frequency's due to the sub spacing. Think about it like dropping two stones in the water some distance apart. the waves colide and some cancel while others add. The stereo effect is difficult to distiguish as the frequency goes down, so low frequency tends to sound mono. Stereo seperation becomes less important and sound wave cancellation can become an issue. Stacking subs is like throwing one bigger stone in the water. There is probably not much stereo information below 30hz so you may find it gives you better impact. You will need to experiment with your room and set-up to see what works best for you.