Sorry for the late reply, had some other matters in life that need attention.
what I designed is a unit that will engage the second voice without the need for a external crossover or Gallo amp. eliminating extra speakers wires and interconnects.
The low frequency range will drop to about 22 hz blending seamlessly with the overall speaker. The speaker is not physically altered in any way,The unit will have 4 wires about a foot long one set L and R for the speaker terminals and second set L and R for the sub in.
The unit itself physical size will vary from about 6 inches x 8 inches , or 8 inches x 8 inches weighing about 1to 4 pounds.
As for my 3.5 s I totally redone the crossover with SOA capacitors and inductors and total rewire but that is costly ….but for me it’s worth every penny.. I have owned the the top Gallo 5ls and I can honestly say the 3.5s upgrade will compete with any 40,000 speakers