Car streaming blues, ain’t marketing great

Harmon Kardon has been making car stereo since at least the early 80's (I use to sell it). It's not top of the line, but usually quite decent. I agree with reubet and M669326 as your first options, if that doesn't work out, I'd suggest upgrading the speakers, Polk, Infinity, many decent brands out there for upgrading. Upgrading the electronics nowadays is either impossible or difficult and expensive for most cars.

I also have XM Sirius in my MB and it sounds fine, not HD, but it rocks fine for road trips. Hope this helps.

Other than checking if you can improve the source, car audio upgrade path is always: power first, then speakers, and lastly sub. A little soundproofing mat I would say is fourth. Just buying new speakers will not work well as stock power is always too little and they stink. I have had cars with HK stereos and always pull the speakers and amp out and replace them. I don’t generally touch the head unit these days but that can be done too but is generally tied into everything so I leave it stock. 

My son replaced the amp in his Subi, it was night and day difference...

One thing to consider is the cost of streaming from your phone if you don't have unlimited minutes as part of your plan. 

I'm shopping for a new car.  Volvo isn't on my list but audi is.   One reviewer said the Bowers and Wilkens stereo on the Volvo is better than the B&O in the Audi which I am looking at.

Sounds like your HK system is the consumer grade system rather than the B&W system which is highly rated.

Jerry