Sorry for departing without thanking all of you for your help. I really appreciate all the advice I've gotten here so far.
Even though I ordered them I decided not to bother with the capacitors, instead I just hooked up my old PC speakers (and subwoofwer) via the ground loop isolator to my DIY RCA sockets.
I've got the manual volume control for the amp/speakers set to about 1/4, which I think gives it the TV an ideal volume range.
It sounded great so I figured job done. Why continue to fiddle about when I've got the desired result?
There is a new problem though...
When putting it together, I made sure to insulate (with insulating tape) wherever the was a join between 2 wires.
I had the speakers on full volume while the TV was unplugged, I noticed the hum got quieter by insulating the joins instead of leaving bare wire somewhat exposed.
I considered the hum at full volume to be acceptable, it was inaudible at 1/4 volume unless I held one of the speakers to my ear.
However, when I plug the TV into a wall socket without turning on the mains / TV's on switch / standby, they hum very loudly.
But as soon as they receive a signal from the TV (when it's on / not on standby) the hum is a lot quieter.
Here's a recording I made with my phone (louder - TV off, quieter - TV on & muted):
If I put the speakers on max and put the TV on mute, I get a bit more hum than if the TV was unplugged, but no where near as loud as when the TV is off.
I'm sure you're wondering if I only notice this when turning off the TV, why don't I just turn the speakers off at the same time?
The problem is the TV and speakers are situated in my bedroom and I rely on the TV's sleep timer.
I've tried this a few times with the speaker volume at the preferred 1/4 level, and every morning I woke up and immediately noticed the faint humming noise. It's quite disconcerting.
For now, I've set the speakers to about 1/8 of the max volume, whereby the speaker hum when the TV is off is barely noticable, but I've noticed if I turn the volume up loud (via the TV's volume control) it get's a bit distorted, so it's not ideal.
Remember that the audio signal from the TV travels through a Ground Loop Isolator. If I remove the GLI and hook the TV directly to the speakers' amp the hum is even worse, and audio from the TV is also affected (distorted and generally a bit rubbish sounding)
I've also noticed the tone and volume of the hum changes slightly when I remove the aerial cable from the back of my TV when powered off.
I suspect part of the problem is the fault of the PC speakers/amp, as I briefly tried some different PC speakers (which I'd prefer to keep attached to my computer) and the hum wasn't as bad.
Any ideas? If the problem is present when it's not powered, would the capacitor trick work?
Everything I've read online suggests it's sign of a ground loop, but then why doesn't my GLI resolve the problem?
Thanks and happy new year everyone.