BH Stereomour, Paramounts. Paramours? The BH amps are parallel feed. So all of them have a parafeed cap to the output transformer. Some also have an interstage coupling cap. So you need to be a bit more specific on what you actually have and which caps you want to replace. The larger the paraeed cap, the lower the bass response goes. If you are only driving the lower mids, then, yes, you can use a much smaller, (lower capacitance) cap.
Miflex caps a HUGE, so getting the requisite real estate may be a challenge.
I would post this query on the Bottlehead forum. You are more likely to get quality responses there, perhaps even from the amp's designer.