Nominal Basket Diameter 12", 304.8mm
Nominal Impedance* 4 ohms
Music Program 1000W
Usable Frequency Range 45Hz-2.3kHz
Magnet Weight 56 oz.
Gap Height 0.375", 9.53mm
Voice Coil Diameter 2.5", 63.5mm
I believe you when you say that they're not as "punchy." You should be running 2 tfm-55xs in mono, one for each pair if you want to even come close to driving them properly. As far as hitting noticably lower, maybe the elimination of the passive crossover has extended the low end response a bit? It seems to me though, that its probably due to a grossly misaligned tuning frequency if anything, wrong size box and ports per new drivers. I'm not an expert in these matters but I do know that ported boxes are specifically tuned to the drivers per cabinet volume.
Klipsch RB 75's
Sunfire 12" TrueSub Signature