JL 300/4 Speaker Distortion

Didn't you get a manual with that amp?  If not, download it http://mobile.jlaudio.com/pdfs/7111.pdf.  "Each input section contains a pair of RCA-type input jacks, an input voltage switch, and an input sensitiviy rotary control".  That rotary control (screw) is the gain.

Did you have this installed?  If so, did you ask any of these questions of them?  The front speaker wiring is not bridged, but it appears that both speakers are connected to the right channel.  Indeed, this is a parallel connection and puts a 2 ohm impedance (I'm assuming those are 4 ohm speakers) on one channel.  But it should not be too low of a per-channel load on the amp.  The wires should be the same way the rear ones appear to be wired.

The wiring is shown in the manual as well as procedures for adjustments.  Two things I'd recommend an immediate upgrade on:  the front set of speakers and the RCAs.

Read the manual completely and let the forum know if there is something that you don't understand.

I'd love to know why the installer wired both front speakers to one channel.   The only logical explanations that come to mind are a bad amp (the other channel is dead) or bad drugs.

As for the gain, flip the "input voltage" switch to "low" and use a screwdriver to turn the "input. sens." dial.  That's your gain control.

Changing the RCA would be last priority.  The highest priority is to put in a pair of components in the front that will not only be at least the RMS power capability as the amp produces, but that will provide a quality sound to go with the quality amplifier.  You may as well have a Profile amp in there at this point.  At least trade up to a higher quality coaxial, but I wouldn't even suggest that baby step.  Get a pair of components that are rated 75 watts RMS or higher, preferably higher.

And, you will need to keep your deck controls at flat or below, nothing in the + ranges.  Do a  search of this forum for "gain" under topics and you will find loads of good info.  But until you get capable speakers into this car, the amp will most likely be sending your Pioneers to an early grave.  And the sound will suck in the meantime.  Setting the gain by ear, using test tones, means that you will be giving those speakers full power using a sine wave, which will be well over their thermal power handling capacity.  At this point, until the replacement speakers are installed, I would say just keep the controls to minimum (no +s) and only adjust the gain screw up from the 0 point enough that the sound is clean when the deck is playing at full tilt, or at about 3/4 of the way.

When you get the new speakers, which should include higher power-handling for the rear as well, you will then have the opportunity to use this system the way it should be used, with complete use of all the controls.  Stay off of any bass boosts, though.  You don't have anything to supply bass (subwoofer) so don't try to make bass happen where it doesn't exist.

Please tell me that you will NEVER go back to that shop again.

Also, getting new speakers is ONE answer to correct the distortion, not the only one.   By lowering the input sensitivity (gain) you can effectively send 50w RMS to your speakers and they should sound fine.   You'll just be using 2/3 of your amp's capability instead of 100% of it.  If you want to use 100% of it, then you need to get  a decent set of comps like Steve suggested.

Personally, I think it's ridiculous that you have to even deal with all of this.  If you bought your stuff off eBay and then installed it yourself (or had a buddy install it), then yeah, this is all part of the learning curve.   However, if you buy your stuff from a shop and pay to have it installed professionally, then you shouldn't have to put up with this kind of aggravation.

Also, make sure your crossovers (filter frequencies) are set on both the front and the rear speakers.  You mentioned that the filter is set to "HP" which is good.  Start with the "95Hz" setting with the 24db slope and then slowly work your way down from 95Hz until you start to hear distortion.  You probably shouldn't go below 80Hz with a 24db slope on those speakers.  You'll have little, it any, low bass, but you won't have distortion either.

Believe me, if one of your amp's channels is blown, you'll know.

I have to tell you though, after re-reading your posts and taking a closer look at your pictures, I'm concerned with what gauge power wire was run from your battery to your amp, how it was run, and what type of fuse was used (near the battery).  This just looks like a complete and total hack job.