Best gain setting for my amps?

The problem should go away after you turn down the gain a little.  Another reason could be a lack of power that is causing the system to shut down.  When does the amp shut down, while riding or while the car is at idle.  If it only does it at idle check your alternator voltage while the system is playing.  Your voltage may be droping so low that the amp is losing power.  A cap will help a little but is not the long term solution.  These amps are power hungry.  I have a 3 farad cap and with my truck running and the music loud it will drop my charging system to 8 - 10 volts. 

As for the bass boost it all depends on what your listening preference.  Remember high SPL's are damaging to the hearing.  Your system maybe louder than you think.  With your car running put down your windows and see just how loud it gets in your car.  Subs move air and with your windows up you are limiting your subs to the amount of air present.

Remember that subs need to be broken in before you can really push them are here their true musical capabilities.

As for the Kicker amp it all depends on what you want to spend and if you think that it will actually be louder than what you have for the price difference.

I have a digital readout on my cap and it dips to the low 11's to high 10's at full load with pounding bass. It's at 14v's+ at normal listening levels. The alternator is 2 months new plus it's pretty heavy duty on these Lexus LS400s. I just need to learn this Ultimate amp as you said. I turned the gain down to 1/4 and I turned the volume up to 80-90% with no clipping or protection cutoff. I think that will solve it. What level should I have my volume at max and how long should I break in my woofers?

Yeah, the SPLs get crazy in my car when the bass is pounding. I can feel a definite sustained pressure in my eardrums after listening to some electronica or rap for awhile. When I listen to the rolling type of bass if feels like my engine is stumbling but it's just the bass. I didn't think the bass could penetrate from my trunk like that but it does. Wow. I've always had hatchbacks with my other ok systems. This is my best system to date though.

I would probably turn that bass boost done, it will probably cause the amp to clip prematurely. I do not believe that the Panasonic head you are using has a non clipping pre-out , play it safe and don't max the volume. If you need to be louder, get that trunk sound deadened asap, trick up that box a little more, have a listen, and then go bigger if you think you need to.

Yeah, the license plate has that ghetto rattle with the bass turned up. These Lexus LS400s have some pretty good stock dampening in them already. Even in the trunk. What should I use to deadened the trunk with? I need to do the box mods too. Maybe I'll do that this weekend? I'll make sure it's totally sealed and then spray something in there. What's the bast stuff to spray in the box?

Yeah, I'll turn down the boost to at least 1/2.

I agree with forbidden, turn your bass boost down to about half. This will cut down on your bass but in about a week or two you will not notice the difference. The break-in period on a sub various by manufacture. Some say no break in is required and some say it is. I think Rockford is like 30 hrs of listening. You will be able to hear the difference in your subs. They will pound lower and louder.

A sub is like a car. It has a motor that has to be broken in before it can be pushed to its' limits.

What type of rap music do you listen to? If you can find some one that has this CD listen to this one song in particular. Artist: Bone Crusher, Track 5 (Grippin' The Grain). This song has bass that will move your subs and help you to make up your mind if you want to change out your amps.

Did you get the whine fixed?

I'm going to Car Toys right now to drop off my car. I'm a hour late, I hope he's not pissed?

Ok, I'll turn the boost down to 1/2. I think this amp has plenty of power, I just didn't know it's sensitivity levels with the gain at what not. You and forbidden have been a great help. It's nice to communicate with somebody that has the same amp as me. I would have to get a quality 1000W+RMS amp to hear anything different I think? These HE2's are rated at 400W RMS each (800W total for both) so I see no reason to get another amp. I think the SQ of this Ultimate DT2-800 must be somewhere on the top end anyway?