Budget 8" Powered Subwoofer Kit recommendations?

I recently picked up a Denon DM-41 mini system for my cabin. It's coming with the stock Denon speakers but I rather suspect I'll use my pair of modified PSB Image 2B speakers instead of the Denon speakers.

The D-M41 is replacing a similar but older Panasonic mini system that I was using to power the PSBs but that unit was a little under-powered for the PSBs being only rated at 20W and doesn't have an output jack for a subwoofer.

I'm thinking I might like to add a small sub to that Denon/PSB system so I'm wondering if someone could recommend a good value not too expensive 8" powered subwoofer kit that might pair well with that system. The D-M41 is only rated at 30W so I don't think the sub will need to be super powerful to match up with the output of the main speakers.

If possible, I'd like the sub to have that auto power feature where it shuts itself off when not in use and comes on automatically only when needed. Power is expensive where my cabin is.

I'd also like to keep the cost down on this so something that's not that expensive but delivers excellent value would be ideal.

I don't need the sub to be able to deliver loud chest thumping bass. I'm not powering a discotheque. I just want it to fill in the bottom end and add to the audiophile listening experience so sound quality and warmth are more important to me than power and the ability to knock lamps off tables whenever the drummer in the recording lays into the bass drum.

Does anyone have any recommendations?
 
I suppose I'd also consider a 10" sub as well. I move thing back and forth from the deck of the cabin to the living room fairly often when I'm there so I don't want to get anything too huge but I could likely live with the extra bulk/size of a 10" subwoofer vs an 8" if the sound is going to be that much better.