Just about any good quality subwoofer would work fine, the likes of SVS are a good choice. If your receiver/Pre-pro does not have a good crossover adjustment make sure the sub has one and some eq adjustments. Subs dont have the same "need to match the mains" issue but all the same you still need to spend the money to get one that blends well.
For a sub with the High level speakers inputs and outputs the SVS PB12Plus is the best choice if you dont require them then I highly recommend the PB13Ultra But that depends on your budget. The PB12 NSD is also a good choice if the PB13U is to high.
If this is strictly for music a sealed sub is also a good choice and there are lots of options however most do not have high level inputs that you require. HSU are also great subs and so are Elemental Design. The SVS SB12Plus gets lots of good comments but it also does not have high level inputs. I honestly think the HSU STF1 is a bit small you wold be better off to get a PB12Plus and you would get the best response, its a fantastic sub and is great for music.
I would actually recommend a sealed subwoofer for your Maggies. I am personally using a Martin Logan Depth with my Martin Logan HT and I absolutely love it. It, to me, strikes the perfect balance between music and HT.
The non "i" version of the Depth should be available for a grand or less. The only differences with the current "i" models are different cosmetics, slightly more powerful amplifier, and tweaked servo control.
When I called Martin Logan about adding a second Depth, I asked them if there was an issue to using a Depth and Depth i and the response was they were so similar that it was not an issue. The same applies with the Grotto as well.
With the Descent versus the Descent i, there are major differences. The Descent has (1) 400 watt amplifier while the Descent i has (3) 250 watt amplifiers. The Depth and Descent both feature 3 woofers spaced 120 degrees apart to reduce resonances.
I have only read good things about Rythmik Audio. I also like that it is both servo controlled and sealed.
That being said, I have never personally listened to any of their wares.
I have both listened to and am a huge fan of both eD and Hsu. Currently, Hsu has the VTF 3 on sale for 639 in the Maple Finish only as well. While both are ported, they are excellent all around Subwoofers.
I would still recommend looking for a used Grotto or Depth, but any of the ones you linked to should work well. Given the Speakers, I might go for the Rythmik because of it being sealed and servo controlled.
I went from a Paradigm Servo 15 to my current Depth and could not be possibly happier. As a fellow Panel Speaker owner, I really think it is worth it to hold out until you can find a Depth or Grotto at a good price.
When my Servo 15 failed, I started a hunt for a Subwoofer that was wide reaching. I came unbelievably close to purchasing an eD A5-350 Subwoofer, but eventually found a Depth available in my area for a ridiculously good price.
Now I'm always trying to save a few bucks here (sub) to spend a few more bucks there (amp) so now I'm looking for input on the Elemental Design A3S - 250 for $350. The Martin Logan Dynamo 10" is in closeout for $450. Both are sealed, the A3S - 250 has a more powerful amp. Hard to find any reviews on the A3S - 250 though.
Does someone know how the LFE input works on the A3S - 250? On the line input jacks it has a Right jack and a Left/Mono/LFE jack but no switch/selector. Does it automatically sense a single input to bypass the sub amp crossover?
When you connect it to the LFE port it will bypass the internal crossover on the eD. eD has a stellar reputation and everyone I have listened to is excellent.
If choosing between the Dynamo and an eD, I would go with the eD. While I love ML's ESL Series Subwoofers, the Design Series (Dynamo, Abyss) really do not offer the same engineering and construction that makes the ESL Series special. In which case, the eD will offer better value and performance.