Anyone have a hiss problem with aftermarket amps in a non-JBL Prius?

I have the bigger Prius V but basically the same Display Audio 6 speaker system as the Gen III Prius and ran into all the same issues you're experiencing. I went with all JL Audio but basically most local shops either recommend JL Audio or Alpine as they're like the Toyota or Honda of the audio world - reliable, well stocked, good value, good quality, good customer service. One of the reasons I went with JL Audio vs Alpine was their amps have built in speaker level inputs, which basically means their own line output converters, so one less component to go through.

My local installer had a heck of a time with the install though. I had it in his shop three separate days (all day each time). After the first install the hiss was really bad and I almost wanted them to take the whole setup out an go back to the factory speakers (but we had to tear out/destroy the front speakers to use the base as a mount, so that wasn't an option). We adjusted the gains, but when it was set high enough that the speakers sound good then the hiss was out of control loud whenever the unit was off, with the gains set to where there was no hiss the speakers were so weak that at full volume it was like how I would set it at regular volume traveling with my wife and daughter (which I don't turn up loud since my daughter is 5 years old). He also suggested a switch to turn off the amp when I wasn't listening to anything, but I refuse to have to do resort to that, plus my wife would say I'm an idiot for spending that kind of money to upgrade the speakers only to have to fumble with another on off switch.

The second time they tried installing different amps (even from different brands), line output converters, adjusting all the gains and other settings, taking out the head unit to look for different outputs. I couldn't really tell if the hiss was less, but I did notice the bluetooth voice was gone. We determined that the bluetooth (and I suspect Nav which I have but hardly use so didn't confirm) was using the front dash tweeter which are on their own seperate channel. The Display Audio powers 6 channels all with ~20 watts each: two dash tweeters, two front door speaker, and two rear door speakers. The shop thought that having too many separate wires for all the channel might be introducing too much noise so they used splitters and only used two channels to power the 4 new speakers I had installed. Well it might have helped the hiss just a little bit, but I lost fade/balance controls and bluetooth voice. I don't really understand how it's setup or works, but my installer was saying when a voice is sent through bluetooth it tries to route it through the dash tweeters first, then after that the sound is routed to all the speakers (or maybe just the front doors), so it actually seems like the sound is coming from all the speakers cause it is, but without the dash tweeters that chain is broken or something. He was suggesting something about installing something with switches or splitters or relays or whatnot so I didn't have to use the dash tweeters, but I didn't really want to waste another day or days and money trying to figure that one out.

The third and last time we went with a higher end amp plus all new speakers all the way around to eliminate that the speakers having any cause in the hiss and back to wiring all four door channels through the amp. I researched specs of different amps and looked for the lowest Total Harmonic Distortion (THD) and best Signal to Noise Ratio. There is also different classes of amp with the newer Class D digital amps smaller while running cooler (which is what you listed that you installed), but I wanted to stick with Class A/B for the richer warmer sound. I went with the JL Audio Slash 300/4 Version 3, as it had the best specs I could find regarding THD and S2N in a 4 channel. We left the factory dash tweeters in place and kept them powered by the head unit so I could keep the bluetooth voice. Upgrading them would be almost pointless because most any high end tweeters are going to require a lot more power meaning a separate amp, and the benefits wouldn't really be worth it. All that research and comparing specs didn't help because the hiss was still there, well maybe it was a little better than the first try but definitely not a lot. At least with the nicer amp there were more adjustments, and after all that research I learned how to make all the adjustments on the amp and what they all do and how you can mess up your equipment if you don't do it right. I was able to find an almost sweet spot where the hiss is barely noticeable while nothing is playing and the sound is very (VERY) good when the music is on and I now will listen with the volume set between 20-30 (out of a possible 63/Max).

Couple of other observations.

Different sound sources sound very different and have much different levels. The CD player actually sounds the weakest and has the most noise for my setup - I can turn it up to Max and it doesn't rattle my head like when I'm using another source. Satellite radio is touch and go for each channel and some are really bad and some are ok. HD Radio sounds very good, I can hear a big difference when the signal changes from regular FM to HDFM. For me at least bluetooth audio sounds really good as well, which is great because I like to listen to Pandora, Songza, and all my uploaded music on Google Music (no need for USB). With bluetooth the sound can get really loud and it's unbearably loud even before the volume is all the way to the Max (which is the way it should be). Thankfully I had my shop install dynamat all over all four doors - so there's not a single vibration or rattle anywhere.

Also, speakers need to have their break-in period, at least I notice it takes a little bit (maybe just a few days or weeks depending on how much you listen to them) for them to perform their best. It feels like they are able to play louder at the same gain settings as they break in, so you may be able to turn down the gains just a bit (enough to make the hiss less noticeable). I noticed this after my install. After about a week or more they seem to sound better, and I'm able to reduce the gains so less hiss. Anyway, with a proper setup you really don't need the gains that high.

With your setup, since your are going through an Audio Control LOC, you may have want to have a professional do the Level Setting like recommended here: http://www.davidnavone.com/heresHow/files/LevelSet222.pdf
My amp had this guide: http://mediacdn.shopatron.com/media/mfg/9013/media_document/live_1/gainSetting.pdf?1342137466
Or you could just go by what this guy recommends: Amplifier gain controls - setting your gain

But even after doing any of these methods I'd still finalize by doing a sound test for hiss, and turn it down until the hiss level is satisfactory. Which I would say is clearly noticeable with your ear about a foot away from the speaker with the engine off but head unit powered and audio off, and practically unnoticeable with the windows down head unit on audio off seated in any seat, and just barely noticeable with windows closed head unit on audio off seated in any seat.

If you do a search just about everyone that tries to install a system with a factory head unit without RCA outs (and even some with RCA's) have this problem.

 

On mine, I had installed a Polk amp, all my stuff works well. all the guy did was run the audio front and rears thru high output to RCA plugs, thsoe went to the same spots on the amp RCA inputs. then they tie into the speakers via the pruis airing harness. I get no hiss, but If I crank the Gain all the way up, yeah you will get hiss from the amp alone from high gain. As far as Bluetooth and phone, those outputs go thru the HU like another radio, cd would. So the installer is not quiet got it right. Mine took about 5 hours to get complete.