Update time ...

Got my tinsel wire fixed.

Paralleled up the voice coil on each speaker, then connected both speakers in series (nominal 3.2 ohm load presented to amp)

Result : Much improved. LPF lifted up to 160Hz without any problems.

I have noticed that a slight distortion is present when cold, but dissappears after 30 mins or so in the car (at which point I can crank up the remote gain pot)

Thanks for the help in solving this problem.