Using absorbent material in the ports isn't the ideal, but if the Eufrodites' way of working isn't working for you and your room, then imo it's reasonable to try to find a solution that is a worthwhile net improvement.
I design speakers too, and most of my designs have user-adjustable ports, because I can't always reliably predict in advance what the room acoustic situation will be like, where the speakers will be positioned, and/or what the output impedance will be of amplifiers they might be used with. I have one customer (a single-ended triode amplifier manufacturer) who went all the way to plugging the ports on his speakers, so they are now sealed boxes. Yeah that theoretically goes against their "way of working", but it's what works best for him, his amps, and his room (speakers up against one wall, listening couch up against the opposite wall, so lots of boundary reinforcement).
As for optimizing the type and amount of damping material, I'm afraid it's a matter of trial and error, and a juggling of tradeoffs.
Duke
I design speakers too, and most of my designs have user-adjustable ports, because I can't always reliably predict in advance what the room acoustic situation will be like, where the speakers will be positioned, and/or what the output impedance will be of amplifiers they might be used with. I have one customer (a single-ended triode amplifier manufacturer) who went all the way to plugging the ports on his speakers, so they are now sealed boxes. Yeah that theoretically goes against their "way of working", but it's what works best for him, his amps, and his room (speakers up against one wall, listening couch up against the opposite wall, so lots of boundary reinforcement).
As for optimizing the type and amount of damping material, I'm afraid it's a matter of trial and error, and a juggling of tradeoffs.
Duke