Well I just adjusted the SLA for each source on my F90BT and it did make a little difference but you can still hear the hissing... but I'll take it, it is a lot better than it was. I guess Pioneer uses the SLA as a gain control, although it is not completely useful +- 4db and even turned down all the way to -4db it still hisses.
I have my factory Bose amp connected to the RCA outputs of the F90BT and do not have any hiss - in order to connect it that way I soldered RCAs on the factory connector wires. The Bose system is a POS but the point is that I do not have an issue even with a low-end amp.
Some amps have a swtich for high / low level input as well as a gain control which can mess with the sound if not set properly - what type of amps do you have?
My money is that something is wrong with your setup rather than with the AVIC. The only true test would be to put a different head unit in there (not another AVIC) and see if the issue remains.
I'm confident with some simple trouble shooting you can get rid of the hiss and enjoy much better sound. Whereas my navi system does not sound as good as my M3 stereo (with all the speakers and amps replaced with aftermarket), it sound A BILLION times better than the factory head unit - and I still have the crappy bose amps and crappy bose speakers...for now 