Hotwiring a Bluetooth speaker into a Bluetooth receiver
if I were you, I would measure the voltages from both your headphones and bluetooth speaker. Most often, a 3.5 mm jack (female connector) output voltage is around 1 Volts. Make sure that the maximum output voltage of your bluetooth system is the same as the maximum output voltage of a 3.5 mm jack. Nothing can go wrong then, I believe. I don't think that (difference in) resistance of the two speakers is important.
Maybe the right output voltage of your bluetooth system is achieved when leaving the pre-amp. I do not have experience with that.
I hope I helped you with this.