I think you should try and get a really excellent fronts and center, they're the most important. It's always upgrading, maybe later you'll want to replace them too. If you go to the speaker forums then tell them what your budget allows they'll be sure to give you the best speakers for the $. Probably recommend speakers like athena and paradigm. I got a cerwin vega package and might have listened to them before I did that because for the money there's better than them. Nowadays there's more power distributed to the rears then before and you can change the volume of each speaker to further asses power distributions. If you get athena's with two 8" woofers, you won't likely need a sub but a sub could always be useful. I say get the best most powerful speakers always, always try to achieve at least as low as say 40-50htz on fronts and rears, well 60htz is not too bad. Anyways, a sub will cover what no speakers can 16htz and up. I'd work on the fronts and center and then see if the bose need replacing, then think about a sub, only you can decide if you need one. Centers take the most use, then fronts then rears. I have a 5.1 and I'm using 4 12" 3-way cerwin vegas for front and rear and a cerwin center (matching) for center. I like to have equally powerful fronts and rears. I think you could get a nice set of fronts and center and then put the bose on rear for now, then you could decide whether to upgrade later. I recoomend you ask the speaker section guys and they'll point you in the right direction. The bose might be good for rears so you might not even want to upgrade, it's your call there, and I'd hold off on the sub unless you want to get one, I've heard good things about sunfire, svs, velodyne, etc. for subs and for speakers athena paradigm wharfedale , well I'm not the expert, they are. If you follow what they're saying you'll be much happier, I learned too late, but my cerwins are a good bang for the buck. The e-series is the best of cerwin vega and you might want them, excellent budget and they are like 95db and kick down to about 30htz. Although they're not as good quality although they do come with a 5-year warranty. I'd say the more you learn the better speakers you'll get. Good luck.
