Use SPDIF OUT from your Onkyo, daisychain to the speakers.

Unfortunately, the while the PR-SC5509 is the top spec Onkyo it seems, it doesn't have SPDIF outputs..

What would that help me with anyways?

Thanks 

That's a first for me - a back panel filled with connections that doesn't have the SPDIF OUT option.

You can consider one of these, or similar:

I know :( I wish I knew then what I do now, before purchasing the Onyko. I assumed as it's one of their top of the line products, it would've had it...

But anyways. I feel I should also explain that the primary sources for the AV Receiver: They are a Satellite Box, and a Media Center PC running XBMC. I think this explains the "hiss" as they are receiving silent noise from these devices, and because the BL5's are almost maxed out, the hiss is very much audible.  The BL5's are not hissing on their own, it's definitely as a result of the silent noise being fed to it, either from the AV Receiver itself, or by the respective sources connected to it .. Which is why I wanted to re-think my setup.. 

I could go with one of these devices that concert SPDIF to RCA/line, however I would be concerned whether or not it would fix this "hiss" sound (can't seem to think how it would though) and whether or not audio signal would degrade by introducing this component into the setup.

Wondering if there is a better setup I could consider that would solve the hissing issue, and how other people have connected and setup their BeoLab 5's with Non-B&O receivers/Amps etc.

P.S. I did make my own Powerlink - RCA/Line trigger box cable, which allowed my Onko to both trigger the BL5's on automatically, and control their volume automatically. This was perfect - auto power on trigger, with no hissing! Except for one big issue - the sound coming from the BL5's was clipping/distorting. I've yet to figure out why, and had to resort to the LINE - LINE connection, whereby the BL5's have to be manually powered on, and the BL5 volume is hard set by myself using Beo4 remote. This results in the hiss which I can hear as a result of me setting the volume on BL5's near max. Like I mentioned before, I wasn't really keen of the thought of leaving the BL5's hard set on half volume to reduce the audible hiss, and controlling audio via the AV Reveiver, because I would always only be using half of the BL5 potential, and also if It was setup like this, and i was to manually control the BL5's volume for when I want more sound like you suggested, then the volume levels would be out as the BL5's would be higher volume than the rest of the speaker setup (BL3's & BL7-2)...

Since my home made trigger solution didn't go great, I was thinking about purchasing this: 

http://uk.bosscom.com/product.asp?id=1082

thanks for your response so far.

Just put them a few more click lower? my BL5 make a slight hissing sound when on top volume, nothing strange, take any amp you want and output full volume without any music and you'll most likely here a slight hissing.

Im running my BL5 to a preamp and have them on maybe 90% of top volume and there dead silent and there's enough volume control to break your ears.

I really dosen't matter where you make the volume adjustment as long as it's to your pleasing.

Preamp running at 100 % and speaker amp at 50 % = 50% of maximum output

Preamp running at 50 % and speaker amp at 100 % = 50 % of maximum output

Yes, but If I put preamp at 70/80%, I want speaker output to then be the same, and not feel like I'm only using the BL5's to half of their potential if that makes sense?

turning them down a few clicks does greatly reduce the hiss thanks, didn't realize they were in fact at max volume... However I still feel the above..

As I noted i can assure you that your not using them below there full potential if your running them at 90%.

Considering that there is no hiss with powerlink then there most likley not running at full volume, otherwise there would be a hiss there to.

Powerlink is just a preamp signal that's volume regulated the same as any preamp, difference is the power on/off trigger...

I hope this makes sense😁

I'm starting to understand you now I think :)

However, by setting them at 90% full volume, and if amp is at  90% volume, then BL5's will be using their full potential. But what happens then when amp (just for arguments sake) goes to 100%, and the BL5's are set at 90%, surely they won't be using their full potential?

Yes there your are correct, but I can assure you that your not going to want be in that room when your at 100%.

As long as you are happy with the sound pressure and your not running out of volume on the preamp it's not a problem.

In my case I usually run the preamp at 70/100 when I want to "party" that's plenty loud for me.