Is it worth upgrading the H&K under-seat woofers to a HiFi system and if so, are they plug/play?
I can only remember that it was described in some thread that the BM-200 under-seat woofer option would be useless for a HiFi system because of the voltage or something. I don't know if that would also apply to the H&K.
Harman Kardon under-seat woofers cannot be plugged into HiFi system because the electrical resistance (ohm) is different. HiFi woofer is 4ohm. HarmanKardon woofers are 8ohm. They are not directly replaceable.
In general, replacing under-seat woofers without adding a powerfull amplifier makes almost no difference. Even with the MatchUP amplifier, the difference between OEM woofers and Focal woofers was not significant for me in my G20. I would advise to invest in the MatchUp amp first. Then replace mids and tweets in the front channels: left, right and center. This will give you a much nicer sound. Replacing woofers makes the least effect.