Crazy 8s Drums

Check to see if it the hiss is out of the coaxials or the components or both...fade back and front. I might blame the amp, but you said you are on the second one and both had the hiss. Perhaps the x-over for the components is having troubles? It is hard to know without the car being in front of us.

Just reducing the variables here. BCBrassard asked the next logical step, your amp gains being set. You stated that you had check RCAs, did you do this by switching to a different set? Are your RCA cables shielded? If a power cable is run through your car alongside unshielded RCAs the power cable can induce noise into the RCAs. This would most likely show up with the hiss changing with volume changes but could perhaps not. The next step would be the amp grounds...make sure they are clean...and at least as girthy as the positive going in the amp. 4g in should have atleast 4 ga out, for example. The connection needs to be tight and corrosion-free. I am guessing you have done 'the big three' too, check those connections for corrosion...just to eliminate potentials. With the HU at 0 volume, check again to hear if amplifier gain change alters the hiss volume. Eliminating variables...

You suggested that the hiss was not volume dependent. Does that mean that when the HU volume is at 0 the hiss is fully audible?

If you are using the ground in the Scion's harness, you shouldn't. Ground it to the chassis. I can't say that it is the problem but it sure could be... https://www.scionlife.com/forums/general-tech-diy-1821/noise-free-audio-tips-47336/

Does the hiss get louder with HU volume? If so, look to the ground from the HU, maybe add another one or improve where it is grounded. The car's wiring harness may not be sufficient ground so you should ground the HU to the chassis and as near to the HU as you can.

I got a XS battery from Showtime and it got sent in the XS single box with a shipping label stuck on it. It was in good shape when I got it but I remember scoffing at it's chintzy packaging considering how volatile a battery can be.

Looks like Kenwood Excelon's are calibrated to not exceed 1% thd at full volume....at the RCAs. The internal amp, not so much... 5 stars suggestion it is the cleanest radio so far is a false claim though. It only suggests that Kenwood calibrated it properly to be at 1% thd at full volume. I am certain that the Pioneer PRS80 and the Sony RSX-GS9 are cleaner overall.

you cant post anymore bmwking..you just hit 666 posts and now you are fully evil. lol

You play the HU at max clean volume, playing track 4 on the disc the DD-1 came with, then adjust gain on the amp up until the distortion light comes on..then back it down just a tad. Then do it with track 6.

Something that may help you tune your sub xovers is knowing that CD, .wav, and mpeg (and most other compressed media formats) have everything below 20 hz already filtered out...and also everything above 20khz. Setting the infrasonic filter to off will likely be the best setting for most music. FLAC, and some other lossless formats, can have lower and higher frequencies in them so if you plan to regularly use lossless media types you may try to dial them in.

Start with frequency all the way up, and with gains all the way down.

You should set HU eq to flat. I know that on most Pioneer HUs, the factory volume max is set very close to where it distorts. Your HU may be like that too.

OpenOffice is free and works very well. Not buggy or prone to viruses etc... https://www.openoffice.org/download/index.html

If you need to finance your soundproofing material you are doing it wrong. Just saying...

When working for MTX, I scored an awesome deal on a scratch & dent Xtant x603. It sounded so awesome and Xtant deserves to be on that list. https://www.mtx.com/x603-car-audio-amplifier