well... to power the speakers you would need either an amplifier. if you kept it 2.0 or 2.1 that would be all that you need to power them and it would work with 3.5mm sources like ipods and phones directly connected. you could also hook it up to a computer and play itunes or internet radio.
a receiver however would offer you more options. you could connect multiple sources up at once, easily connect more speakers, calibrate the speaker delay, as well as a multitude of other options.
a receiver is going to typically cost about $200-300 brand new (you can find cheaper used). as far as what would work with your system you want to look at the spec sheets and see what is capable of powering around 140w @ 4ohm. if memory serves me right.. i believe that most 100w@8ohm receivers typically have specs at about 150w@4ohm so thats a good starting point.
speaker sensitivity also has alot to do with it. the lower the speaker sensitivity the more power they require. a good little discussion on that here
http/www.whathifi.com/forum/hi-fi/what-drives-4-ohm-speakers-well
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as far as amplifiers are concerned...i did a bit of searching around but didnt find anything quite as you needed it. this one
http/www.ebay.com/itm/PYLE-300W-2-X-150W-2CH-DIGITAL-AM-FM-STEREO-RECEIVER-AMP-AMPLIFIER-PT560AU-NEW-/330837633962?pt=Receivers_Tuners&hash=item4d0775abaa would have been perfect if you had 8ohm speakers but since you have 4ohm speakers is overkill and you could blow them. for your current speakers i'd look for something closer to 150w x 2ch @ 4ohm.
however... that amp would work perfectly with something like the sony f5000p floorstanding speakers which went for about $100/pair brand new a few years ago. i found some used here for the same price you paid
http/www.ebay.com/itm/Sony-SSF5000-Floor-Standing-3-way-Speaker-48003-/321392307540?pt=Speakers_Subwoofers&hash=item4ad4796d54 if i remember they were 150w @ 8ohm speakers.
we have them in the other room running on a receiver without a subwoofer and they get LOUD as in loud enough for an electro party even on an amp which isnt capable of pushing them to 150w. since they have woofers you really dont even need a subwoofer.
however, you already have some towers...which is a dilema.
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i'd still say a receiver would ultimately be a good idea if you wanted to attach subwoofers and more speakers in the future though. however, again its going to cost about $2-300 for something half decent unless you buy used.
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anyways... just a few ideas...