^^ what numeric said!
get a LOC, HEAPS of different brands out there, from cheap ebay chinese to expensive brand names ones (Rockford, Alpine, Stinger, and the Soundstream you mentioned)
then there's signal processors...once again, all the big names seem to have one. (I have no experience in these, just what I've read on them)
If you search the audio section (and may be VE section) for LOC or signal processors, you should find enough responses to people who them, the brands they've bought, or have just gone with a simple LOC.
I put an Aerpro AP-304 in the other day, mainly because it was a good enough brand not to be crap and it's priced really well:
AP304 Audio level converter | Aerpro Vehicle Specific
I've read others getting the Jaycar LOC or even ebay LOC's and having no issues at all.
The other idea would be to get an amp with high level / speaker inputs, and running the speaker outputs from the back of your HU to your amp (nowhere near ANY power source, otherwise you'll get an alternator whine through the speakers). Details on which wires:
https://forums.justcommodores.com.a...-unit-wiring-digrame-ve-sv6-2008-model.184803
Sorry I'm not able to comment specifically on IQ system, as I don't have one but from all I've read on JC members putting amps in their VE's (IQ or not), either high level input into amp, LOC or signal processor are you options with a stock HU.