Those Sony speakers are 3 ohm (the center is 6 ohm). The lower the ohm rating of speakers the higher their volume will be given the same amount of power.
Those htib type systems use low ohm speakers in ordrer to get the highest volume output from it's internal amp as possible. That's why car stereo's use 4 ohm speakers. They have small amp sections due to space limitations in the dashboard.
Unfortunately, those htib's also use very low quality amplifiers in order to keep the prices low. These low quality amps produce an enormous amount of distortion (10 %). It's this high level of distortion that damages speakers, not too much power.
The biggest issue with replacing the front speakers is they use a proprietery connection. I suppose you could cut the wires at the speaker end and they should work, but you won't know for sure unti you hook the replacement speakers up.
I would look for used speakers that came with a different htib system that are rated 3-4 ohm as "regular' speakers might put too much load on the Sony's amp section. Same with 6-8 ohm speakers. You would have to crank the volume up, introducing all that distortion again, in order to get a normal amount of volume out of higher ohm "regular" speakers.
Or, replace to whole system and start over.
Not what you wanted to hear, but's it's the price you pay with these all-in-one systems.
"Everyday room": Panasonic 58" Plasma, Dish HD DVR, Pioneer Elite vsx-23, BDP-23 BR, dv58avi universal dvd player, Paradigm Studio 20 V1, CC-450, Dayton HSU-10 subwoofer.
"Movie/Music room": Toshiba 65" DLP, Dish HD receiver, Marantz 7005, CC-4003, BD-7006, Polk LSI25's-LSi7's-LSiC, 2 original Dayton 10" "Mighty-Mites" subwoofers. (subject to change without notice).
Also have MB Quart Vera VS05 +.....too much to list. Help me.