Go for a Kenwood eXcelon XR-5S This is what i have in my vx for the front speakers , back speakers , and for the sub in the spare tire area (still in progress ) I got it from ebay
This one is fit perfectly under the front seat , with a capacitor , or a High Current Car Audio Power Cells
The amp dimmensions are :
• Width: 9-1/4” (235mm)
• Height: 2-3/16” (55mm)
• Depth: 7-9/16” (192mm)