there's one altec lansing stand alone subwoofer for sale, i think the model is XA 301 or somethings like that. looks like a bomb, u can add it to ur ACS45.1 easily.
but for yamaha woofer, u might be a bit troublesome to set it up. as u must has an crossover before sending the low frequency signal to woofer.... not sure the woofer alone has built in crossover or not.there's a lot of things u have to take in count when u buy those theather woofer. in connecting it i mean.

Those AL subwoofers are pretty old and are discontinued a long time ago, I never heard them before but I believe your CS21 will pawn them easily because newer model normally is a step forward than the older models, right? If CS21 also you feel tak cukup ooompphh, you should be getting a much better subwoofer.

Well, most HT subwoofers have a crossover inside whereby you can control the cut off frequency of your satellites (or speakers in layman's term), so you can choose say 40HZ, that means 40HZ and below, the sound will be handled by the subwoofer, the satellites handle the remaining frequencies. It is adjustable but honestly, HT subwoofers a little bit mahal lah but will give you the bass that you want, that type of solid and hard hitting bass that can cause jakit jantung, you actually feel the bass, with my Yamaha subwoofer at 40% of the volume, my table and chairs already vibrate liao and including the water in my aquarium too. Yeah, it's that powerful, with 40% of the volume and the bass level set correctly, you'll in for a surprise, that's something pc speakers cannot compare but it's not fair to compare this way lah because my Yamaha subwoofer is already the price of two sets of CS21 and you're talking about a subwoofer only and nothing else. There are cheaper models from Yamaha, as low as RM500 plus but I never tested it before, supposed to be an add on to a Plasma or LCD tv whereby the bass is severely lacking. So far, I never seen people using HT subwoofers for pc lah but the wiring no problem one, you can always modify it, 3.5 mm mini stereo plug to RCA.

No lah, my HT subwoofer is not for my comp, my comp is in my room, connected to my AL MX5021 but it can be used for a comp lah, the cable can be bought anywhere, less than RM8. Not very expensive, you can DIY yourself but I think no need lah, buy outside much easier.

Your budget around 3++ , right? Well, I'll search around for you to see if there's such a subwoofer suitable for your budget and no need to say lah, I know what type of bass you like.   

By the way Bro Narkon, my friend said Edifier not bad, he was referring to this model, http://www.inter-asia.com.my/Review/X600%28HWM%29.jpg but I dunno the price lah, he said the bass is good. You can listen to it in their Plaza Low Yat showroom. 

Yalor, I honestly don't encourage you to sell the speakers lor because when it comes to second hand ones (even if you bought it yesterday), buyers will keep forcing you to lower your price sampai you feel that it's not worth selling, rather throw the speakers into the sea for Nemo or Sharktale to listen than to sell them like Petaling Street prices.

So, where's your location Bro? Are you from Melaka? Don't get angry lah, Edifier X600 also cannot find in Penang but Logitech Z-2300 got la, I've tested it, it goes pretty loud, bass is satisfying but I didn't test with my own music lah but the bass definitely there but what I dun like is that it doesn't have a remote control especially when it's priced over RM500 plus. 

seriously, you can find HT subs in Melaka Raya. You know where is the old Senyum? There's a car park beside it, and beside the car park there's an electronic shop selling speakers etc. They are more towards stage speakers and performance equipments like lightings, but they do sell HT subs. Get one active sub and get proper cables lo.

And by the way, its not a good idea to have 2 subs. Firstly, the dominant sub will overpower the weaker one. Second, different subs different sound signature, you mix them up and you'll get rojak instead. Thirdly, 1 + 1 don't always equals to 2. 2 subs means you have 2 drivers pumping the lower frequencies. At the point where the sound waves bump into each other, it will cancel each other off and you'll actually get milder bass that what you current has. Still remember a chapter on "gelombang" in your high school physics?

Unless your speakers are 2.0, then yes, getting a sub will allow you to obtain a fuller sound spectrum, but 2.1 + 0.1? I'm pretty sure it will be a waste of money. If your CS21 really doesn't suit you, just sell them off lo. There are a lot of speakers that produce heavier bass, let alone the price might be cheaper than your current CS21. Name a few: atp3, Eacan, Klispch GMX2.1...

RM380 for a 2nd hand set? Most people will rather pay a little bit more to get a brand new set with full warranty instead. As for clear sound, it is because your bass doesn't overwhelm the mids and highs. You crank up the bass knob, it eats up the details. There's always a balance there.

See the main point everyone?
Now i know which brand speaker you should go to already.
Go for Logitech's dude.
Another word for Logitech's speaker is a boom boom speaker.
Hope i got this right. 

sunauto: May our MX5021 rules