There's a lot that goes on in a room besides the speaker and especially the amp. To a large extent, the sound you're getting is dependent on the room, and what you're doing with the room (speaker locations, listening position, acoustic properties of the wall surfaces).
People who've become adept at picking out which frequency ranges are doing what (boomy, low or high in level, etc.) can tune their system more by ear, by playing tracks they know well and have a good reference from some other system.
I am not such a person, so I've tried test tones and graphing the response from sound level meter ($40) in Excel.
You learn a lot from such an exercise: playing the front three speakers by themselves without a sub, without a crossover, one speaker at a time, etc. You might see large peaks or dips in the response, and if you know what the speakers are supposed to do anechoic, you can see what the room is doing to the sound.
You can see the response graph change as you move speakers around, closer to walls, farther, in or out of room mode positions, etc.
So, your lack of bass may be due to the sub (50-60hz) or the front speakers (80-120hz). Or, maybe ANY speakers and subs placed into those locations would have the same problems, because they're yielding dips in the response at those locations. Only through changing the positions of speakers and/or listeners would this get resolved. Also, what about the movie soundtracks sending bass to the center channel? If your center speaker isn't up to the task (and this is the first or second most speaker (the other is the sub) in home theater), then maybe you're missing bass here too.
It's tough to judge speakers' capability when pushed hard (louder) by a frequency range spec (-3db 36Hz to 20kHz), because maybe its woofer cannot produce that 36Hz to 400hz at high volumes. If the tweeter has no problems, then what you get is more treble at high volumes, with less bass, and distorted bass, or your amp clipping creating more distortion and grating harshness.
It's also tough to judge sensitivity. When Klipsch says their speaker is 97db efficient, they don't tell you at what frequency. Does one watt produce 97db at 36hz (which will tax the woofers)? Or just the 1,000Hz they happened to test and report at? And just because it can produce 97db with one watt of power from one meter away, will that scale properly up to 75 watts to yield 116db?
Although I'm suspicious of Klipsch's claims of efficiency for the non-horn-loaded woofers, it does seem like two 8" woofers would be up to the task. Then again, there's a reason why JTR makes the Triple 8 speaker.
Personally, if I knew there was a lack of response at, say, 80Hz, I'd see if moving the crossover down helps, so that the front speakers are handling that range more. Or move it up, so that the sub is handling those frequencies.
Also, your sub could be in a bad spot, and moving it a few inches or feet, or to an entirely different location in the room, will make a huge difference.
While you could play around with how it sounds when you move it around, and do the sub "crawl test," it's so much easier to measure and see the response in the graph.
I know it's incredibly alluring to most guys to throw more watts at the problem. It's a shiny black box with lights. And surely 110 is better than 75. But in reality, that increase in wattage gives you only two more decibels of output
If you're playing music in stereo mode, and the Klipsch even without the sub are not giving you chest slam, I'd first look to placement, and then if that doesn't work, only THEN look to an external amp. But first upgrade that sub for home theater!
Next step: find out what the problem is (where are you missing what frequencies). And it looks like you need a sub upgrade regardless (less boomy, and get a good response down to 18Hz or so).