The Jeff-xover for cradeldorf will be this one.

Altec Lansing 604B Phase Correct Crossover

This has been modified (16 ohm version) to suit the 604B, and hence will also be good for 604C.

Is the DCR of a 604B/C is at 8.3 ohms ? Jeff's site says nothing about this. Also there is no imp-graph. Only the HF side is there. He only mentions it as a 16 ohms driver. After GM's hint in this thread earlier about Jeff's work, I have been searching everywhere. Finally I found one page from somewhere on the net that I have lost trace of, and it says the 604B to be of 10.59 ohms, and the 604C of 11.10 ohms. The measurements were taken in the yr 1971. The Altecs are towards the bottom of the page.

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Cradeldorf had got these refurbished at GPA. I think they can confirm about this parameter issue. Otherwise the xover may not match. Ofcourse the best will be to measure them up.