dB stands for decibels, which is a measurement of sound pressure levels (SPL's), on a scale of 0 to 194. A zero value on the decibel scale represents the weakest sound audible to humans and sound intensity increases in correspondence with numeric values, the relationship among the values on the decibel scale is not linear but algorithmic.
Therefore, the simple assumption that a sound with a 50 dB level is twice as intense as a sound with a 25 dB level would be incorrect. Rather, in a perfect world, each three decibel increment affects a 50% change in sound pressure levels. Thus, a 3 dB drop reduces sound exposure by 50%, while a 6 dB drop reduces exposure by 75%. Though reducing the decibel level produced by a sound source from 80 to 77 may not seem like a major change, it would actually represent a 50% reduction in audible sound.
So to make a long story short(er), let's look at your RTI10's. With a -3dB at 35hz, these speakers will put out 50% less apperent sound pressure at 35 hz than at 38 hz. The corssover is set at 12 db/octave. An octave is double the frequency, so 20 hz to 40 hz is one octave, while 1Khz to 2Khz is also one octave. So if you lose 12 db/octave, your spl'd should be down 12 db at 20 hz when compared to 40 hz. In other words, while your speaker is putting out 89 dB at 40 hz, it's only putting out ~ 77 db at 20 hz, a 12 db drop is a drop of about 130% in SPL. Remember, 10 db is twice as loud, so 89 db is twice (100% louder) as loud as 79 db.