As a side note, it's possible that the output is either a fixed volume output, variable with your TV volume setting, or toggleable between the two via a setting. Given that it says 'headphone', it's most likely a variable output that auto-mutes the TV speakers which is probably preferable.
It looks like a Polaroid 32GSR3000, which if so means that the orange RCA connection next to it labeled COAXIAL is a digital output, so you could perhaps also use that with a coax-to-RCA adapter (something like this, although I can't vouch for this exact model's quality). But a regular 3.5mm to dual RCA adapter like /u/vandal_taking_handle suggests is a simpler and cheaper approach.
The sound you describe can be caused by a weak signal connection. Verify the 3.5mm jack has a good connection and the cable you are using is not the issue. MB42x sound great and the APO fix makes them even better. If one of the speakers was bad you would hear a tonal difference between left and right channel.
I like the Peace EQ interface for APO. It makes it a little easier use.
This is what it should look like. I dunno if you’re in the US but Amazon would be easiest. Also any electronics store is likely to have them.
Like this guy? https://www.amazon.com/Monoprice-111567-Desktop-Headphone-Amplifier/dp/B00KVVX2QW
I'm wondering if I need something in between as far as features go, as I'm realizing that I'd really like to be able to run a basic microphone input through the device as well. Just a 1/8" or 1/4" type connection, nothing fancy like the XLR connections. Wouldn't ever be used for anything more than online voice communications.
I replaced the fans yesterday with 2 of these units.
https://www.amazon.com/gp/product/B00KF7P34G
I cant believe how quiet these are! I thought I screwed up and didnt have them wired right but they were running. Completely silent.
If keeping the remote control is a priority, some people have had success with line level converters. Other option is to use the tape output in conjunction with a pot in a box like one of these, but you will lose remote volume capability.
There's a few voltage powered outlets/power strips available, but they are geared more towards the semi-standard 12 volt trigger.
http://www.amazon.com/Niles-Black-FG00242-Voltage-Triggered/dp/B00086IDDC -- I think 12 volts only
http://www.amazon.com/XANTECH-AC1-Controlled-Discontinued-Manufacturer/dp/B000NU0T62 -- this one takes between 5 and 30 volts, so you'd only need a small relay to step up the 3.3 volts and kick out a higher voltage.
There's also the Wink compatible outlets and stuff like that. (Which totally boggles me that you can get a wirelessly controlled outlet for $40, but a simpler hard wired one is $100)
It sounds like getting a line level output that I can plug my iphone into is a good idea. Would something like this be good? Or am I going down the wrong path? thanks http://www.amazon.com/Schiit-Audio-SCH-13-Passive-Preamp/dp/B00K6Q2A4C/ref=sr_1_5?ie=UTF8&qid=1452203068&sr=8-5&keywords=Preamp
Take a look at smart power strips like this one. Without cracking one open, I bet there is a hall effect sensor on the master outlet that controls a bank of relays on the switched outlets. Just connect anything like a receiver or TV to the master and the speakers to the switched.
Not sure about that Onkyo, I would go with a surround receiver for bass management incase you add a subwoofer later. Just based on the information available online this Denon looks like it might work out.
If I was in your position I would really consider the JBL LSR305. It sounds better near field, and has the same bass extension as the B5, although the B5 can play cleaner at the limits of it's volume. For a front end the Emotiva DC-1 is going to be the best option. Although it is super pricey, it has a remote control and lots of inputs. If you are using a computer as a primary source the Lexicon alpha would be a cheaper option, but lacks the remote and only has USB and one analog input.