Jacking the seat up for needed clearance

You are only talking about 2" max height increase... I don't think 2" higher will make a difference. For example, I am 5'8", person X is 5'10". That is a 2" difference. Person X will be more likely to get their neck snapped in a roll over accident? The other issue here is that your seatbelts are attached to the floor / frame, not the seat. You may want to make sure that you will have enough length on the seatbelt receivers to still be able to buckle up. In other news, have you concidered moving the box forward? If your rear seats slope upward from rear to front (as mine do Chevy ext cab) you can slide the box away from the rear firewall to achieve a little more room. How are you dealing with the excursion of the sub not hitting the bottom of the seat? Or is this the problem you are concerned with? I ask these questions as I am in the process of doing the same thing as you.

Update - last night I realized a very easy way to "lift" the seat enough to clear my box. Instead of removing the seat assembly and installing washers, tubing, spacers, etc on the factory stud I simply folded the seat up and placed about a 1/2" spacer in the spot where the seat rests in the "sitting" position. This gave me an extra 1 to 1.5" of clearance at the front edge of the seat. The seat is now tilted up at a very slight angle, is the same height as before, and is still safe to sit in. Even the seat belts are still usable. The spacer I installed is just a piece of MDF I had laying around. I intend on drilling a hole to install a steel spacer painted textured black to match perfectly. Hopefully, others may find this helpful! Mike

Shltplease - I am very concerned with the excursion of the sub hitting the bottom of the seat. That is the reason I need to lift (now tilt) my seat up a little. Also, Polk recommends about 1 to 1.5" of air space around the sub so it can push the air out of its way at full excursion. FYI - I am using a Polk MM2104 (10" sub) Mike