Help Diagnose what's wrong with my Subs

So I have been noticing latley that my subs are producing very little bass and absoultley no punch

My set up is 300/2 bridged to 2 JL Audio 10W6's which are about 4 yrs old

The amp is run through a line out converter through my stock MACH 460

It almost sounds as if the subs are crosswired, but I konw they arent. The amp is still ok, it gets power, its grounded and the subs definitly move, just don't produce any bass. They also are producing large amounts of distortion at low levels of volume. At high levels of volume they produce no bass at all

I originally thought the Subs were blown, but after examining them it does not appear to be the case. Maybe my amp is fried (after a 3 hr drive on a really hot day) or maybe the line out converter is shot (three years old)

I took the box and amp comletely out, re-wired everything and I still have the same problem....so I am just not sure

any help would be appreciated, I just dont want to start randomly replacing things

alright thanks for your help, unfortunatley I do not have an ohm meter so I wil have to see what i can do about that

I pushed down on the woofers, no rubbing, they felt pretty normal actually. So i am assuming now our sights are set on the amp granted the ohm meter reads normal

the subs are run in series...not sure if that would make a difference

is there anyway to tell if the line out converter is actually bad..or do i just have to order a new one and see if it works...