Need a very good amp for my Focal sub

Are you experiencing cabin gain 'boom'?  http://www.diysubwoofers.org/caraudio.htm 

It sounded better when you reduced the gain?  Step one in amplifier setup is to take time to set the gain correctly, have you done this?  You could be hearing the amplifier clipping. 

I could be way off base here, but you might have done yourself a huge disservice by playing the sub in your home first.  One, the noise level in your home is probably very low compared to the car.  Two, you experience gain like auto cabin gain when in a home, causing the sub to sound deeper.  I know my home sub sounds incredibly different depending on where in the room it is placed. 

The car environment is a different beast.  First, I don't know that I've ever heard a sub sound great looking right at it outside with the trunk open.  Close the trunk and tune your sound for the drivers seat position from the drivers seat.  It takes longer, but who cares what the sub sounds like from outside the car with the trunk open?  Sub bass produces very long waves.  If my understanding is correct, by standing too close in an open environment, you won't hear much in the way of low bass.  I don't have a strong physics background here, so someone correct me if I'm totally off base.  The length of a single cycle of a 40hz wave is around 28 feet.  You can hear this in a room, or in a car, standing much closer to the source than 28 feet because the bass wave can reflect off walls (fold) to get to you and complete the cycle.  So, if you're listing to a sub with the trunk open, outside, standing closer than 10 feet away, you won't hear much in the way of low bass because the waves have nothing to fold off of.  Similarly, if the same wave is bounced back a perfect 180 degrees from the source it can actually cancel the original wave, and you won't hear much of it either.  This cancelation happens often if the sub bass waves cannot escape the trunk.  Are your seats too thick, etc?

My advice: 

Someone let me know if I'm off bass (heh).  

heh heh I like that...I think I'll add that to my vocabulary...And forget what stuff sounds like in your home, you are in a car environment, kfr01 made some good points.  If you can find someone with a CAR system you like the sound of, emulate that setup.  If you keep testing out the car components in your house, you'll never find nirvana.