I think you got your questions answered...but just in case:
I used XLR-RCA converters at the JBL (these are not cables, just XLRs plugs with Female RCA jacks on them), so then just needed a standard RCA - Mini cable to plug into the TV. This approach keeps the 'special cables' pile minimized, and lighter weight
Hiss is in the ear of the be-hearer. I can't hear the hiss on either the 305s or Kali IN-8s at more than 2 feet or so, and not at all when playing. YMMV.
One thing I DO like about the Kali's (same designer as the JBLs, BTW) is that they shut down when not 'in use'.
I don't think the latest JBLs have that feature? Not a huge deal, either way. (Kali's IN-8s are in a higher price range, too, though you may want to consider their smaller 305 competitors.)
Also, instead of getting a sub-woofer, consider moving up to the larger JBLs (or Kali's) - the 8" versions are not terribly more expensive, and may eliminate your need for a sub-woofer (or at least postpone it
.
My TV also has the headphone vs. speaker selection; not sure what it changes, though in any event driving a low impedance (headphone) output into a high impedance device (power speakers) is ok electrically anyway.
Regarding room EQ, probably overkill for TV audio - but I'm sure your Sony has a vast variety of tone controls?
The JBL sub (310) is a great idea - you'd just connect it in (depends on the sub) from the sub and then to the JBLs, or just parallel the inputs (Y-cable adaptor). (Just checked - I have a JBL PSW-1000, and oddly it only has RCA inputs, and no pass through, but also speaker level (!) inputs, but also have a Yamaha sub that has RCA inputs and pass-throughs....)
You haven't mentioned if you are going to use a Blueray player or not - if so, I assume you'd just 'HDMI' that to the TV and be done with it.
Regarding adding a DAC - I actually bought a Topping DX3Pro for exactly that purpose, TV Optical -> DAC -> Speakers....it even has a nice remote and BT, etc.
But I found that -
FOR TV & these speakers - it didn't sound dramatically better, and now I had to use TWO remotes for TV watching - one for channels, the other for volume/mute.
Bleh. Topping went back in its' box to find another use case.
Of course, if you move into surround sound (unless your Sony has multi-channel analog outputs - nope) you'll be looking at least for a HT preamp processor with optical and HDMI inputs (which you could still use small powered speakers with)...and opens up a whole new world of 'cross-fire' and 'confusion'!
Finally, FWIW, I've just been
amazed at the performance/value of these powered speakers - though not even close to the 'fidelity' of my large main system, for TV or desktop use they are pretty stunning, and now they are adding more useful features, like Bluetooth, etc.
PS. Final thought - these things don't have grills, so you need to watch out for gefinger-pokenings - an audio buddy of mine found some super cheap fans that came with grill guards that fit my 305s - with a bit of 'plier' love - just perfectly!