Best multi-purpose subwoofer

Hi guhlamr,

     I also have an Oppo BDP-105 in my system. Excellent and versatile machine that has exceptional audio performance.  
     I also use a 2TB external hard drive that contains ripped copies of all my cds and some other hi-res 24/bit/96Khz Flac files (great minds?).
     Your description of hook-up details looks good to me with one exception:
" the only manual change between HT and dedicated listening will be to remove the XLR inputs from Simaudio and connect the RCA input from Marantz every time I set it up for HT (have to borrow the 2 channel amp). I can use a switch box for that eventually. "

     Why not just connect the front L+R mains outputs from the Marantz to the L+R inputs on the Simaudio amp permanently?  You're only playing music or HT and never both at the same time, right?  This would eliminate any need for unplugging/plugging of connections and switching would be automatic and seamless.  
     If there's no switch on the back of the Simaudio amp to select XLR or rca inputs, this indicates that both inputs are active and the unit will just amplify the signals that are inputted.  I doubt it would attempt to amplify both XLR and rca signals if they are present at the same time.  But it really doesn't matter since you only play 1 at a time (music or HT) there will only be either XLR or rca signals inputted to the amp at a time and not both.
     I also wanted to suggest a change to your HT setup  that I've made in my system that you may not have yet considered.  This is just an optional change to your system mainly focused on simplification and streamlining.
     I believe you could remove the Marantz AV7005 from your system without a performance penalty.  You're currently using two high quality prepros in your system, the Oppo and the Marantz.  Unless the Marantz has a capability that the Oppo lacks that I'm unaware of, there really is no need for this redundancy.  
    I think we both use a 5.1 surround for HT in our 5.1 HT systems (5.4 with the Swarm?).   You could connect your 3-ch amp directly to the Oppo 105, it has a very high quality surroud sound processor that decodes up to 7.1 ( contained on a separate/isolated audio board using multiple high quality and expensive Saber 9018 dac chips), the cross over frequency can be set on the Oppo and its LFE output can be connected to the LFE input on the Swarm amp.
     I eliminated a Parasound AV2500 prepro from my system and replaced it with the Oppo 105.  I thought the surround performance improved without a downside but YMMV.  You could experiment and decide for yourself.
     For streaming, I'm just using Spotify on my laptop connected to a separate dac,3 watt class A headphone amp and good phones.  But I think I should probably be receiving advice on this rather than giving it.

    Overall, you're plan sounds very good to me with the exception of switching inputs on your amp.

Tim 

+1 for Rythmik.
My FG12 is a servo-controlled, paper-coned, 12" sealed sub. Very fast and musical, it marries really well with my Maggie MMGs for music and is all you could ask for in a HT application as well (certainly for my modestly-sized listening area of ~ 24’ x 15’). Has the connections you want, is powerful, infinitely adjustable, and the dimensions are almost exactly what you listed. Very reasonably priced, I’ve always considered it a superb value for the money.
I even bought it here on Agon!

I love the REL subs.  I use my G2 primarily in a 2 channel system for classical music.  Some rock.  It is also the base for my 7.1 system.  It mates surprisingly well with my Magnapan 3.7s.

Of the 3 subs the OP is looking at under his $600 budget, the Earthquake is an absolute beast compared to the others. I owned a couple of those before and that's a lot of subwoofer for $600. 

I have had 5 different SVS subs, none of which are still here.  I had one of very expensive Paradigm subs which was too boomy for me, so it's gone.  The room I am in is 30x30 with an 18 foot high vaulted ceiling, a mezzanine level overhead walk way, stone under it and glass on the opposite wall.  The other walls are made up of cabinets, stereo gear, book shelves.... its a mix of everything, but for some reason, things just work well for 2 channel stereo which is my main use.   I am currently using three subs: Axiom Audio EP800, EP500 and a Bryston Model T sub.  The Bryston is made for them by Axiom too.  Honestly, there is no comparison between the Axiom products and any of the SVS subs I used including their flagship Ultra SB16.  Hands down the Axioms are far more powerful, smooth and just plain musical.  For your sized sub required, I'd suggest the EP500.  And pay no attention to the power ratings on subs.  The SVS's were rated at some huge number of watts, but after two of them burned out and I took them apart to inspect; all I can say is the amps are in my opinion too light.  The Axiom amps which are rated at a much more conservative output have monster linear toroid power supplies.... the dynamic  head room is immense and you cannot clip these drivers.
Check them out, I think you will be very happy.