first, for you and anyone who's advising.
you'll find the manual for your system here.
that suggests that your two front speakers should be as such.
100W + 100W (8Ohms) Page 46
(the page after this confirms that your speakers (the ones that came with the system) Page 47
8ohms each, and presumably have a power handling ability to withstand 100W RMS (each)
Your rear speakers are 12.5w + 12.5W
rated at 16Ohms with a power handling of 12.5W
the speakers (according to page 47) -again original as sold with the system
16ohm impedance rated for power handling of 12.5W, (each)
Surround sound centre speaker is 25W at 8Ohms
the speakers page says that the original speaker was 8Ohms presumably power handling of 25W
at the bottom of page 46 it goes on to say what speakers you can connect to this system.
Headphones can be as low as 8ohms, (these would be the old fashion can headphones that actually had speaker cones in them i'd imagine), but more realistically would now likely be smaller headphones that have a much higher impedance.
the front speakers should be 8 - 16 Ohms (and no less)
the rear speakers should be 16 Ohms (and no less)
the centre speaker should be 8 -16 Ohms, (and no less)
As discussed you can make up these impedances however you like.
(connect 2 4 ohm speakers in series / Connect 2 16 ohm speakers in parallel for 8 ohms, connect 4, 4ohm speakers in series for a 16 ohm load, or 2, 8ohm speakers in series of have a single 16 ohm speaker).
If you put a lower load than is expected on the output then usually bad things can happen, for example if you put a 4 ohm load where there should be a 16ohm load then in theory the amp will try to push a lot more current into the lesser sink, an things will get hotter, and likely burn out faster, or distort a lot more.
There are two suggestions that I'd make for getting more bass.
first the cheaper suggestion.
put a speaker in the centre out, and see if you can adjust the EQ for this speaker alone. if you can't do this then see if you can get a bass pass filter for this speaker. so that the centre speaker is only delivering bass.
and the more expensive/better results option.
Use your AUX out, get a separate EQ or filter unit, and a separate bass amp.
then attach your pass speaker to a separate amp that you're using to drive that.
And the third option, (which is again expensive).
Get some decent speakers, ones that are built really well and a set of decent speaker spikes to put the bass into the floor where the effect will be greater.
Lastly, with the power your system puts out.
the manual says PMPO of 1500W, that's quite a crazy figure, this is how they get away with writing that.
also THD, (which is total Harmonic Distortion) is 10%!, perhaps I should have just held down the shift key and wrote that the THD of your amp is !)%!
(as in a nasty four letter word!).
I'm assuming that you must notice this when it's turned up full?