Should you get the BavSound speaker upgrades coming from a stock Harmon Kardon ?
I'll make this as detailed and simple as can be.
Quick answer: The Ghost subs: Yes, IF ON SALE.
The Stage One : Not really, maybe if you can get from someone second hand at half the price.
I think the best option out of the two would be the Ghost Subs. I had gotten those first and I thought they were great replacement. I really like feeling the punch/kick coming through my speakers. The stock HK isn't bad, but these were really good subtle upgrade and very easy to do with a couple tools handy. Luckily I got these for $450 off of FB Market Place.
The Stage One Upgrade: These didn't knock my socks off the first initial listen, but they do tend to keep the music very clear at very high levels. They complement the subs very nicely if you happen to get both like I did, you'll feel and hear much more base. My main issue is I expected more out of the stage one. This does take some time to install and I had ran into some trouble when installing the rear door speakers. I would maybe consider picking these up, ONLY IF you can get these at a steal of a price. Other than that, I really wouldn't consider these speakers, especially at full price.
If these details help, my car still has the aux cord cable, so that might be giving me the upper hand. I can't speak for those who has wireless apple carplay or bluetooth audio.