best 4.5" midrange drivers under $1K

Try 6,5" Lowthers and reconsider paper. It's very fast, hopefully not much faster than yr tweet. The drivers mentioned above, excellent though they are, are very difficult to work with and, I speculate, mismatched sonically with yr alu & fiberglass.
I would further recommend you reconsider the request for a 4,5" to play 150Hz and look for a bigger boy: say, 8".
Regards

I totally agree with Gregm. I would be tempted to double up on most 4.5" midranges when working to 150 Hz. I am skeptical that even the accutons could work well that low even if doubled up at all but the most modest of volumes.

Pulp Paper is underated. It is very fast. It is also resonably damped inmternally (the big issue you get when you go metal/ceramic is "ringing" and the need to consider notch filtering to reduce its audibility).

yes I agree with the ringing of the non paper, but have you tried excels? They certainly dont hv common weakness compared to other nonpaper..hence my search yet on nonpaper

Accutons cant go low unless the 6"-7"..even 5"one cant go as low as others in my experience

but have you tried excels?

No. But I am old school and a woofer or mid/bass with a voice coil diameter barely bigger than a typical tweeter kind of rules them out for me. You are talking thermal compression big time, IMHO - even at modest levels.

but try excel whenever wanna opt to metal without ringing with still gives you involving and sweet sound

yeah it's 4in afterall..i cant go bigger cause dont wanna to rework much on enclosure and my listening space is limited if not small