i havent heard the HK system, but i constantly hear that the subs aren't great. also, if you have the regular system, there is no stock location for the HK subs. it really wouldn't be worth the modifications you'd need to make to get them in there.
300-400 bucks is a tough budget. i'd recommend looking on ebay or craigslist for some deals on used equipment. be picky with your brands though as a lot of the used stuff i see for sale is crappy and meant for huge, crappy sounding bass.
if BSW has a cheaper price on an open box setup, then that may be best. if not, then first focus on replacing your front speakers.
i was looking for a cheap system as well. i settled on using a 4channel amp, can be had for $120 i think. replaced my front speakers for $120. and put a 10'' sub in the trunk also powered by the 4 channel. the rear speakers are stock. the sub and box can be had for maybe $150+ used subs might be your best bet for a cheaper price. look for single 10 inch ones. anything else will need too much power. with the additional $20 for an LOC and $40 for a amp wiring kit your looking at around $450+ thats about the cheapest system i can think of thats worth doing. i didn't include the cost of the speaker mounting plate adapter i needed as my speakers didn't match screw holes, plus th minor costs for crimp connectors and speaker wire that i had laying around. i used all alpine parts. R series speakers up front. 70w x 4 amp, and S series 10'' sub. it sounds very good, and the sub will hit pretty hard if you want it to. the sound is accurate, and fills the car. exactly what i was looking for without spending loads of dough. install was pretty easy and i did it all in one afternoon.
i'd imagine my system sounds better than the BSW setup, as i used an aftermarket amp which will get much louder than the stock one, and the bass from the sub is nice. but if your looking for something extremely easy to install the BSW i bet will sound good. the stock speakers really suck, and tend to squeal and sound really bad at higher volumes. any replacement is worth doing.