The F22 audio systems-hifi and HF-have several issues. First, the speakers are junk. The magnets are small and the cones are paper. The second issue is the amp. It's an adequate amp but it's not tune-able. Lastly, the digital to analog converters in the head unit are mediocre. Retail prices for 2 channel digital to analog converters on
www.mouser.com range from less than $2 each to more than $60 each. The differences include sampling rate, conversion algorithm, bits, etc.
The first thing is to create a budget. Then start with the speakers. I recommend that you buy a pair of sails for the component tweeters. Then, select your speakers. Eton, Audiotec-Fischer, Focal all make direct replacement speakers for the front pair. The rears aren't as critical. I went with JL 2 way speakers.
The next step is to buy a tune-able amp. I went with the Audiotec-Fischer MATCH 7UP BMW amp. It's tune-ability is daunting. But, I took my time and learned the software and experimented with it. I also added the Bluetooth module to the amp. I tried the USB module first, but my iPhone was finicky and sometimes would stop interfacing with the amp. The Bluetooth module has the latest Bluetooth and format capabilities. JL also make a tune-able amp but it's not plug and play like the Audiotec-Fisher amp,
Lastly, chuck the subs under the seat. Like the other speakers they are crap. I'm still researching these. But, the Audiotec-Fischer amp can be tuned for which ever subs you install.