SQ amps

An amplifier's RMS rating can exceed the speakers RMS rating IF YOU SET THE GAIN CORRECTLY.  If you set the gain incorrectly, then you could run into some real, real serious problems.  It's all about setting the gain right.  This can sometimes be a good thing because you wok the amp less, but you MUST know what you are doing.

150 watts is just +3 db from 75 watts.  It depends on the overall efficiency of the system, the installation itself, the specific vehicle, and a lot of other things to be able to tell if this increase in volume would be painful.  The benefit, though, would be that you're not pushing your gear to the point of clipping (turning the volume all the way up to the "magic number") when you want to be loud.  You could keep the volume knob down just a bit.

And by driving 150-watt-rated speakers with 75 watts and less, you'll be staying well away from the speakers' thermal handling capacity.

I'm going to recommend

Eclipse Premium Digital Amplifier Series;

Altomobile class d amplifiers; and

Xtant 1.1i + Xtant class d subwoofer amplifier.

I'm incredibly convinced that full range class d or t amplifiers are the future of car audio.  While these brands aren't the cheapest, the money saved by not having to upgrade your alternator, wiring, and other power system components is great.  High efficiency low heat designs were made for car audio.

Prior to moving to the Xtant 1.1i mono blocks for my front channels, I was bi-amping my front components with a class ab Xtant.  I was bridging an Xtant class ab 2-channel amplifer to my subwoofer.  Car batteries didn't last quite as long as I would have liked them to and the sound seemed as if the dynamics were pulling back ever so slightly when I turned the volume up.  Moving to class d not only eleviated all these issues; I now feel no need to upgrade my power system, but it also gave me some great separation and added some great midbass tightness.  The highs are slightly brighter with the 1.1i than with my old ab, but overall I am much happier with an all class d system than my old class ab rig. 

btw:  Onedonut:  impressive system list. 

While we're on the topic, some brands that were once respectable but I'm unsure of now -

Soundstream, PG, PPI, Xtant, HiFonics

Have all these guys gone the route of RF or still making the good stuff?

I've had nothing but a great experience with Xtant.  I've owned 6 Xtant amps in two years.  No problems, great sound. 

I hear they don't hold up to SPL abuse.  They weren't designed to be SPL amplifiers.  The power supplies are regulated more tightly than your average SPL amp.  They are well built, but aren't ready to be in a car with very high sound pressure levels.  This is fine with me, because I couldn't give a rats ass about SPL.

The Xtant 1.1i received good reviews.  Example:

http://caraudiomag.com/testreports/0308cae_xtant/

The reviewer found the build quality to be excellent and the sound quality (measured and subjective) to best many class ab car audio amps. 

While I don't think the Xtant name has the same weight it carried in the mid 90's, I'd still place it above the average offerings from HiFonics, PG, PPI, and Soundstream. 

I say average because each of those companies makes some nice products at the top of their price offerings.