Same thing happened to me with MH6 (Singapore)
So here's the thing - I have a mild vision impairment and having bold fonts throughout the entire OS helps a lot. I've always used "MyFont" to change the UI font to Akzidenz Grotesk Bold (gorgeous font, really)
So anyway the MH6 update broke MyFont - something KNOX related I suppose - it couldn't apply the font changes anymore, even though it worked perfectly fine on stock unrooted MG5 firmware
So I dug up my old XXMEA firmware, Odined my phone... then I had the worst "bricking" experience I ever had
a. Couldn't make calls (black screen)
b. No sound output from speakers whatsoever (no boot sound, no touch tones, the phone doesn't "beep" when adjusting the volume, etc)
c. Erratic connectivity to H+
d. WiFi unable to turn on
e. Very unresponsive UI
So I updated to MH6 through Kies (on another PC), wiped data, and then it refused to get past the "Samsung Galaxy S4" screen - it just hung there. I flashed broodplank's vanilla 4.3 over it, and managed to at least boot up (albeit with the same issues). THEN I noticed that the baseband was still MEA - which meant that Kies kinda failed as it upgraded ONLY the main firmware but not the modem - perhaps that's why it wouldn't boot up?
After many hours of trial and error - I managed to get my phone back in working order.
1. Flashed XXUDMH6 modem
2. Downloaded XXUDMH6 (MM1, my telco) from samsung-updates.com, Odined it
But it's kinda disheartening that Samsung "locked down" the phone with these latest updates.. what a shame. Definitely getting a Nexus when my contract is up.. even though here in Singapore they're overpriced :P
Hope this helps anybody stuck in the same scenario - the gist of it is.. if you want your phone to work properly, you can't downgrade your firmware. Kinda like Apple.
If irony were made of strawberries, we'd all be drinking a lot of smoothies right now - Newscaster Ned, South Park