Hardware upgrade recommendations

So here's the deal. My recent purchases [two earphones, the MEElectronics A151 & VSonic GR07] have inspired me to upgrade my home phones [Sennheiser HD 497 @ HD 280 Pro, my first forays into higher end headphones]. I've pretty much decided on the Beyerdynamic DT880 [250 ohm version] as my main home phone to complement my portable Panasonic HJE900.

Trick is, my home listening is done through my computer, which, while it is a more recent purchase, still uses onboard audio [because I have the Logitech Z-5300 speakers hooked up, which isn't worth getting a separate card for]. I'm thinking the Asus Xonar Essence STX should be a sufficient upgrade in terms of being able to properly drive the DT880s. There's still another problem, though: the Logitech Z-5300s.

The front right speaker is starting to go; it is justification enough for me to want new speakers. I'm thinking bookshelf speakers, though I do want a separate subwoofer, as I am used to having one. I was originally thinking of the Audioengine A5s with the matching sub before realizing that it would run an unreasonably high price [Amazon has the combo at around $700]. I figured there were cheaper, better alternatives available. Newegg had the Polk Audio Monitor40 Series II on sale - $150 + a promo code that knocks $42 off the price. They went real fast, though; they were in stock earlier in the day, but are all out of stock now. Still, I'm a bit out of my league, here.

I'm sure there are good/better options out there available. I'd prefer to not push the price too hard [since my current influx of audio equipment is costing me]. Let's assume I'm going to push it through a Asus Xonar Essence STX card, but I'd prefer to not have to run an amp or anything like that; space & budget limitations imply a need for simplicity. So minimum, I'd like a a pair of speakers, though I'd really like a sub to go with it [I'd probably wind up hooking up the z5300s to my TV for a cheap, minor upgrade in TV sound]. Pricewise...I'm generally willing to pay a little more if it means getting better bang for my buck. So really I'm looking for value here, though ideally, I wouldn't be paying more than USD 150-200 [for a pair of speakers; more if a sub is included].

I should note that in terms of music that I listen to, my listening spans all types...though if push comes to shove, I suppose I could say my music skews towards hip-hop & all kinds of rock & metal. Additionally, I do watch quite a few movies & TV shows through my computer in addition to music, so please keep that in mind.

Thoughts?

Suggestions for a sub?

Infinity Primus P162 [which actually looks like it's been replaced by the Primus P163BK, which presumably is pretty much the same] paired with the Dayton SUB 120 sounds like it might be a decent setup, especially to start with [since there's room to grow].

One more question: what amp should I pair this with? Again, I'm going for value here. Also, it'd be nice if I could also use said amp as a headphone amp in addition to the speakers.

Hooking all this up will be new to me, but I assume I should be able to figure out the connection wires & connectors fairly easily....

So in theory, could a HK 3490 hooked up to an Asus Xonar Essence STX powering a Dayton SUB 120 & Infinity Primus 163BK speakers also be able to properly power a 600 ohm headphone [IE the 600 ohm version of the Beyerdynamic DT880]? I have doubts as to whether the Xonar Essence STX could do it by itself, and I want to have the option of properly running high impedance headphones if I wanted to.