Heh, I'll keep it reasonable then
Since you were originally looking for a sub, I'll keep the emphasis there and spend less on a center. With that in mind, for a center channel I'd take a serious look at the
Wave Crest Audio HVL-1,
Ascend Acoustics HTM-200, and the
Hsu Research HB-1 Mk2.
I'd probably go with the Wave Crest out of the three, it's the cheapest and when designing it they concentrated on making the mid range as good as it could be which would be beneficial for the center duty.
This leaves around $350-$370 left for the sub. In this price range the
Hsu STF-2 is would be my choice and is one of the best subs under $450 out there (excluding sales and clearance).
With the HVL-1 and STF-2, that lands you at $450 and with shipping will probably put you around $500, this will be a significant upgrade over the Insignia.
If you want to be less optimistic about budget and get an even better sub, there's the
SVS PB-1000,
Rythmik LV12R (which can be purchase through Wave Crest or Ascend), and the
Hsu VTF-2 Mk4. Out of these, I'd probably slightly lean towards the Rythmik though any is a solid choice. Here you're looking more around $650-$700 (with the center channel) but these will be a clear step above the STF-2.