-3db limits for dummies

can someone explain what these numbers mean to the lay person trying to set up an HT system. upper and lower -3db response...

I am tyring to set my 3 channel HT system to sound better and I don't really understand the best way to optimize crossover freq.

Bottom line I moved and some of my equipment is out of order, so for time being I'm stuck with Onkyo txsr705 receiver as amp, in room that is very reflective and it sounds wierd. sounds like tin... I'm sure my other AMP and my old big center provided a lot of depth and better sound...but for now this is where I'm at...
Hardwood floors. Polk RTI 10 right and left. new in wall center Polk 265 Timbre matched . spikes on speakers *should I take them off on hardwood?

Center Speaker info
91 dB
Recommended Amplifier Power
10-200 watts per channel
Total Frequency Response
Upper -3dB Limit
25 kHz
Lower -3dB Limit
45 Hz
Nominal Impedance
8 ohms

Total Frequency Response
Upper -3dB Limit
Lower -3dB Limit
Nominal Impedance
8 ohm
89 dB
Subwoofer Crossover
125Hz, 12dB/octave low pass
Mid-High Array Crossover
2.7kHz, 12dB/octave low and high pass. 125Hz 12dB/octave high pass
Recommended Amplifier Power
20-300 watts per channel

I also have a sub PSW 505 I have not hooked up because my preouts on my receiver are blown...
any suggestions on how my system should be set on receiver before PSW to optimize sound and after I hook it up.

Total Frequency Response
23Hz - 160Hz
Upper -3dB Limit
Lower -3dB Limit
I don't know the exact scientific answer, Almarg can maybe speak to this, but I think that the +/-3dB is a range within which a speaker's frequency response is tested. I have also seen a +/-6dB range, on the specs for the Quad 22L2, and have read that this wide a range means that the extreme points of the frequency response can actually vary widely. keeping the specification within +/-3dB results in a more dependable rating of the low and high frequency responses.
I think.
Crossovers roll a speaker off above or below a given frequency. The knee in the response is where this rolloff begins and the -3db point is where the crossover has knocked down the output of the driver by 3db, which also happens to be the half power point for electrical power.

Many designs strive for the higher freqency driver to be 3db down at the bottom of it's passband, where the lower freqency driver is 3db down at the top of its passband. In effect, the higher freqency driver is handing off lower frequencies to the low frequency driver and the higher freqeqencies are routed to the high freqency driver.

Ultimately, IMHO, you are best setting these things by ear as the slope of the crossovers, driver artifacts, amplifier impedance, room gain and many other factors make it very hard to set crossover points by the math.
thanks.... as confused as ever... but appreciate it...
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.
I have a very similar situation to Riasillo, I have an Onkyo 609 avr connected to polk lsi9 speakers above tv with hsu research stf1 sub in a big entertainment center with turtle back ceiling. I am just doing a lcr with a sub configuration and looking for a center speaker. Jmcgrogan2 thank you very much for your explaination because its very clarifying and helpful. I am trying to figure out what speakers I can use with my lsi9's on a college students budget. Really no question here but just saying thanks for your explanation it helped me.
best bet might be to find a used pair of lsi9s so your center is timbre matched and use one for the center channel (or both if wired parallel, i think that's how you would use 2 speakers for center, but not sure... ask that question in speaker section in a new thread for answer if you can find a pair of the lsi9s.. probably find them for sale on polkclub site for sale by members there..
Then in future, if you ever want to upgrade front, you can get new LRC and use your 4 lsi9s for 7pt surround.

just a quick thanks to jmc for explanation, appreciate the time to explain. I guess what I'm confused about is how to set the dbs for each frequency i can change in my amp set up. Right now they are all at 0db. I have like 8 crossover points I can change the db on... never played with it before... any suggestions on starting points..
Riasillo, why do you feel compelled to change them from zero? Just because you can? Most amps (like mine) have no built in equalizer, so 99% of us here listen to 0 db across the frequency spectrum all the time.
I don't know if you can adjust the sound to sound less "reflective and tinny". You may be able to do something by dropping the upper midrange or high's by 2-3 db. It sounds more like you need some sort of room treatment. Sound bass traps, absorption panels, carpet, over stuffed furniture, something along those likes.
I was just looking to change whatever I could to get better sound...but good to know you don't usually mess with those settings...
at this point I'm awaiting new receiver.
my audyssey won't work right on onkyo. guessing more than pre outs got fried...

I think I'm going to take the onkyo to the gun range and give it a thorough field test...