So you are changing from using the Onkyo's internal amps to the 7700, correct? You should certainly hear some improvement there.

Frequency response just relates to the range of sound that the speaker can reproduce - there's little to do with how it will interact with an amp, although lower frequencies would tend to be easier to reproduce with additional power. The range of human hearing is typically defined as 20Hz to 20,000Hz, although our high frequency hearing degrades over the years.

The crossover frequencies again relate to the speaker design itself - those are the frequencies at which the speaker's internal crossover network divides the work up between the 12" driver, 6" and 4" drivers, and 1" driver.

Sensitivity does play a role in how a speaker interacts with an amp, as does impedance. A 91dB speaker with an 8 ohm impedance is what I'd classify as a pretty reasonably "well-behaved" speaker - it should be able to be driven to the limits of its ability in a typical sized room with an amplifier comparable to Outlaw's Model 7125, although the larger 7700 will give you more power reserves for those moments when you might need it. It's common to see speaker sensitivities (also sometimes called efficiency) that range from as low as 84dB or 85dB up to the mid-90's using typical cone drivers. If you move to horns, you can see efficiencies as high as 100dB to 110dB, at which point it takes very little power (just a few watts) to produce very high volume levels.

Let us know how the 7700 works for you, and if any questions crop up. Oh, and be prepared - that's one heavy amp!