Yup, it's a tight fit under the passenger seat, but it is possible. Using an xacto knife, cut some of the padding from the back of the carpet, and attach the amp with zip ties or velcro. My Alpine 4.150 is attached with velcro. Hacking the carpet under the seat is what made the difference for me.
As a rule, I'd suggest that the upper dimension for an amp that will fit is an Alpine 4.150. I 'spect googling "alpine 4.150 manual" will get you a .pdf that will show you those dimensions.
Good luck.
geosynch