[mdnky]

To be the most accurate, you'd need to know the actual DC Resistance (Re) values of the tweeter. Sometimes the MFG will provide this in the TS parameters, or you can measure the Re with a DMM. Unfortunately Infinity (Harman) isn't known for providing that data freely, so best we can do is guess. Honestly it should more than do for your purpose, since you're just using it as a bass blocker and not an actual crossover. Its basically just half of a 1st order crossover.

Assuming you've got the Infinity REFERENCE 375TX tweeter from the REFERENCE 6530CX set, they list* the 'nominal' resistance of the tweeter at 3ohm. You wanted a XO point of 2500Hz, so we'll use roughly 1/2 that for the blocker. You'll need a ~44uF non-polar capacitor for each tweeter.

Parts Express has 47uF 100V Electrolytic Non-Polarized caps for $1.76/ea which will work fine for your application, but unless you order other things to get the free shipping option buying the 10pk from Amazon is slightly cheaper (and...you'll have spares).

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47uF 100V Electrolytic Non-Polarized Crossover CapacitorElectrolytic non-polarized (bi-polar) capacitors are perfect low-cost solutions for use in passive speaker crossovers. These capacitors feature axial leads, a 5% dissipation factor, and are rated at a 100 VDC working voltage which equates...

www.parts-express.com

Amazon.com: 10 pcs Non-Polarized Electrolytic Audio Capacitor (100 VOLTS) (47uF) : Industrial & Scientific

Amazon.com: 10 pcs Non-Polarized Electrolytic Audio Capacitor (100 VOLTS) (47uF) : Industrial & Scientific

www.amazon.com

6db filter:

         1
------------------- * 1,000,000
[(2∏) * Re * Hz]

         1
------------------- * 1,000,000
6.28318 * 3 * 1200

    1
---------  * 1,000,000
22619.448

0.00004421 * 1,000,000 = 44.20974

* https://www.infinityspeakers.com/on/demandware.static/-/Sites-masterCatalog_Harman/default/dw9e84a64e/pdfs/Infinity_Reference_Speakers_Spec_Sheet_English.pdf