A good subwoofer will have a large magnet and strong frame on the speaker, as well as thick walls and bracing for the cabinet, which adds weight. A light weight sub means a cheap speaker and cheap cabinet. You get what you pay for. For reference, look at the
SVS PB-1000Pro for comparison. That's an example of a good quality sub. The sealed version, the SB-1000Pro is smaller and cheaper but does not go quite as deep. Looks like prices are going up in October.
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If you ever decide to go that route, SVS allows free return shipping so you can try them out.
PartsExpress and Monoprice are on-line retailers. I would not expect to find any Black Friday deals on their cheaper priced subwoofers. You might find a name brand sub for half off on BestBuy but you need to carefully compare the specs to see if it's truly a good deal. Ported subs are larger but generally perform better than sealed units. Sealed is only recommended if you have to have something smaller and less expensive.
For your surrounds, you can stick with Polk if you prefer to keep to the same brand. It's not necessary with surround speakers unless you plan to listen to 5 channel music. Surround channels carry mostly effects so it's not as important to timber match them as it is with the front 3 speakers. I see that
BestBuy has the Polk Signature Series S15. There is a
detailed analysis of the S15 on Audio Science Review. The speaker is on the bright side due to accentuated highs, but has decent bass for its size and does get a recommendation in the end (although it is a marginal recommendation). Considering that it's only $230 for the pair I think it would be tough to beat in that price range. Anything cheaper and I think you're then getting into territory where good sound quality will be lacking.
MonoPrice has the K-BAS for 30% off. Their speakers tend to offer very good value for the money (particularly the THX speakers). You could consider going for some less expensive surrounds and putting more into the subwoofer budget.