[Ron Ramirez]

Yes, you can use an S-7 (Philco 60 speaker) to replace an SB (Philco 84 speaker). The SB, as you have noted, has a lower current capacity and therefore the size of the field coil is much smaller.

I'm frankly surprised that Philco, the constant cost cutters, even made a different speaker for models 84 and 37-84 when the S-7 could have been used instead.

The field coil of the S-7 isn't that much larger than that of the SB so you should have no problem installing the 60 speaker into your 84. The frames of both are the same size.