what cap value for 2 ohm 6x9's?

Glad you're getting benefit from using this forum.  It's a great place.

It's all about prohibitive cost.  You weigh the price with the value it provides.  Most people would install one 250 uF non-pol electrolytic and be done with it.  I would use the bundle of three that I recommended, using the higher quality poly cap as a "bypass cap".  I wouldn't spend the money required for all poly for what you're doing.  And that still might be overkill in this situation because

• you don't know what the actual impedance is at the crossover freq.  It might be much higher than 2 ohms at 150 Hz.  There isn't the supporting test data that you need to have to know what it is.
• using the calc, you can see how the cap value changes according to the impedance at the crossover point.

You'll probably not notice a difference in sound in this application using the different quality caps.  You might have to end up experimenting a bit with cap values because of the question of the impedance.  A non-polarized electrolytic cap is relatively cheap and would be the type you would want to use if you had to switch out the value (had to buy replacement caps).  So, you're best bet is to just put a 250 np cap on each one and see what it sounds like.  If it's not solving the problem, switch to a 150 uF np cap and try that.

OOOPS  I'm going to look this over.  I linked you to a calculator for 2nd order crossovers .

Here's one:  http://www.mhsoft.nl/spk_calc.asp#crossover  First order crossover is indeed over 500 uF at 2 ohms, 265 at 4 ohms.  But I suspect at that freq the impedance is rising rapidly and is well over 2 ohms.  I don't think you would have to be anywhere near 500 uF for your cutoff target.  Try the 250 first because I suspect the impedance is probably closer to 4 ohms at that point.