Was it really an impedance mismatch?

I have been using Ayre preamps for many years. With multiple outputs (balanced and single ended), I always used the balanced outs to my amplifier. Contemplating the addition of a subwoofer, I asked Ayre if there would be any issues using the Single ended outputs of their preamp to connect to the low level inputs on the subwoofer. Ayre said it could result in a noticeable effect on the high frequencies but I might not notice it. So I ran RCA cables to the sub while still using XLRs to the power amp. To be honest, I was so focused on the bass and getting the sub dialed in that I didn't pay much attention to the possibility that the overall balance of sound was different.

Fast forward a few months, I had become less satisfied with the sound of my system. It seemed to lack HF extension, clarity, and dimensionality so I began searching for a new preamp to replace the legendary K-1xe.  On a whim, I added a second subwoofer to the mix: the left channel RCA cable connected to one sub, the right channel RCA connected to the other. OMG!  There was HF extension, clarity and incredible sound staging in which instruments were brought into better focus! The I remembered the warning from Ayre about using the two preamp outputs simultaneously, maybe something about impedance matching issues.  Did two powered subs change the impedance such that they were a better match for the pre?  The subs are cheap ones made by Dayton Audio and never really impressed me.  They have 12 dB per octave filters and even set to the lowest crossover of 40 Hz they seemed loose.  They bled higher frequencies regardless (I have a CD with pure frequencies from 20 to 120 Hz).  So I found these in line low pass filters and I bought the 50 hz ones which also have 12 dB per octave roll off and placed them between the RCA cable and the sub input jack. They were only 28/pr and my rationale was if two subs and their loads could improve my sound that significantly, maybe these low pass filters would be even better. Besides, they are filtering everything above 50 Hz before it even gets to the subwoofer.  I have to say, the bass is tighter than ever and the rest of the musical spectrum sounds wide open. Is this even possible? Does it make sense? Was there really an impedance mismatch between the preamp and the sub which only became apparent after I added a second sub?  Maybe someone with an EE background can explain it better.