After upgrading from BIAB2018 to BIAB2021, when I play any existing MGU file over MIDI to my Tyros 4 keyboard then the playback is muffled. If I export the Song as MIDI, and transfer it to the Tyros 4 using a USB stick, then it plays fine on the Tyros 4. And then when I play it in BIAB2021 again it will be perfect and it will continue to play perfectly until I load another song on BIAB2021 or until I reload the BIAB2021 app. After which, if I simply (on the Tyros 4) load that exported MIDI again then BIAB2021 plays perfectly again regardless of which Song is loaded.

The only MIDI cable that is connected to the Tyros 4 is the MIDI IN - so there is no MIDI Thru.

Any suggestions most welcome please as this is a pain.

Off the top of my head:

Have you tried the USB connection to your computer?
Have you tried the other MIDI input on the Tyros?
Have your tried to change the gain on both BiaB and the Tryos?

Could you upload a sample of the muffled sound so we could hear exactly what you hear?

Hi

Not that I think this is your current problem but some thing to keep in mind.

When using an arranger keyboard or Clavinova CVP, as a midi module to play back midi files, even if that midi uses the higher bank panel patches lsb 112 113 etc.
Although it will play the correct voices they wont sound quite so good sent to the keyboard this way say from a DAW to the midi sockets, as they would have, had you selected them from the keyboard panel which would send audio DSP effects to match the voice, to the keyboard at the same time.
Can result in an unexpectedly thinner sound.

Just a thought
Mike

Sounds to me like that MIDI file you load somehow tells the Tyros to enter a different Mode (which is why it then responds to BiaB differently).
In other words, the problem may not be BiaB behaving differently, but rather the Tyros behaving differently when it receives data from BiaB. I doubt loading something into your keyboard changes BiaB.

In your example the common thing is loading a MIDI file in the Tyros, then both the Tyros and BiaB work fine, so I'm wondering what MIDI message in that file is triggered that changes the Tyros somehow.

'GM On' mode message maybe?
Or Performance .. or?

There was a bug in earlier builds of 2021 that caused BB to send "MIDI CC 74=0" which basically sets Brightness to zero. That was fixed in patch 804. Try downloading the latest patch from here: https://www.pgmusic.com/support_windowsupdates.htm#809