I just received the JBL SB400 from Crutchfied yesterday and spent my first evening with it last night. If you'd like some quick initial impressions of the JBL to help in making your decision I'd be happy to post them for you here but, if you're intending to use your Polk sub anyway, then the JBL with it's own bluetoothed sub would be unnencessary really. What you may really need is a speaker bar (a passive soundbar). The Definitive Mythos line may be a good choice in that case. Crutchfield carries that line as well.
On the other hand, if you have another use for the Polk or you want a more wireless set up in your room then, maybe getting the JBL is an option for you. Let me know and I'll post a few initial thoughts tonight if it's helpful.