[ab initio]

I recently received the Samson MediaONE 4a powered monitors as a gift. I have to say that I'm really happy with them, having not personally owned any speakers beyond a few old stereo shelf systems. Right now, I've got them set up in the same room as a Sony STR-K7100 5.1 system, and I think the Samson monitors best the 5.1 system in terms of sound quality; however, there is one exception: low bass.
 
I'm honestly surprised how well the Mediaone 4a's do in terms of low frequency response considering the 4" speakers; however, they just don't muster enough to thump in the lowest octaves  to---for example---do justice to the cannon blasts in the telarac recording of the 1812 Overture.  (But boy-oh-boy do those speaker cones really move during that part of the piece!)
 
So, I think I should supplement the monitors with a subwoofer. The general idea (I think) is to get a powered subwoofer with a crossover. In this case, the sub would need to take line-level input and pass the high frequencies to the Samson MediaONEs at line level, while powering the low frequencies.
 
It seems there are many sub choices with many different sizes (e.g. 8", 10", 12", etc...) and various power ratings. What should I look for in an appropriate pair to my Mediaone 4as?
 
some facts on the MediaONE 4a: 

1. Drivers: 4" copoly + 1" silk dome tweeters
2. Passive crossover "design for linear response from bottom to top"
3. rated FR 65Hz--22kHz (claimed spec).
4. The back of the speaker also shows a FR graph and it appears that the MediaONEs are Flat +/- 3dB from 100Hz--20kHz.
5. Rated power 20W+20W (total 40W) (claimed, I dunno if it's RMS or peak)

 
One thought might be to look at Samson's own powered sub offering: Resolve120a (front firing, i think)

1. 120 watt low frequency amplifier (claimed, I dunno if it's RMS or peak)
2. heavy-duty 10" transducer
3. 40-180 Hz. (claimed, I dunno if it's +/- 3db, 10db, etc...)
4. active crossover (have no idea how the phase is affected.)
5. Phase switch

 
Another option might be something like the Dayton SUB-XXXX line (e.g., SUB-800)

1. Power output: 80 watts RMS
2. Box design: Bottom-firing Ported
3. Frequency response: 40-140 Hz 
4. Crossover frequency control: Continuously variable from 40 Hz to 140 Hz @ 12 dB/octave
5. Phase switch
6. Perhaps a bit more challenging to setup with powered monitors 

 
I honestly don't know how the price or the performance of these subs compare. In fact, there are a lot of things I don't know, and I would be happy if someone would care to offer advice:

• What other subs might/should I consider?
• How much should I expect to pay? (I would want the price to be order-of-magnitude appropriate for the quality of the monitors)
• What size transducer should I look for? is 8" fine? what are the benefits for getting larger?
• What is appropriate low frequency range? is 40 Hz low enough for music? movies? (I think Telarc 1812 has goodies down to 20 and maybe lower... )
• What is appropriate high frequency range? How does the high frequency range relate to where I would want to set the crossover frequency and/or my monitors frequency response?
• What kind of cross-over network should I look for? Passive or Active? minimum phase or maximum roll-off or other?
• At what frequency would I want to set the crossover network for optimally matching with the speaker's response? Namely, if the speakers have a corner frequency at 100Hz, would I set my crossover there?  lower?  higher?
• Do I want a 12 dB/octave crossover? 24 dB/octave? Does it matter and/or do I choose based on the low-end roll-off of the speakers?
• Front-firing vs down-firing?
• Ported vs not?
• Are there any questions I'm not asking, but that I should be considering?

 
 
I really appreciate any help or advice that you might offer! Thank you!
 
Cheers