Hello,
I have a soloist midi track that makes a Vsti play with a trombone. In BIAB the sound of the trombone is satisfactory and quite realistic. When I export the track in Wav I have a quality drop that turns the sound of the trombone into a much uglier sound.
What measures can I use in BIAB to obtain the export highest quality that is so important in the brass sound?

Rather than render the file, record the audio output using Audacity or a DAW. You will be directly recording what you are hearing. It is done in real time so a song that is 3:10 in length, will take 3:10 to record plus you will have to trim the beginning and end of the recorded track.

Well, it's been said that digital is digital.... so with that in mind....

If there's a difference, it could be the player. So, take the exported wave and play it in BB or your DAW. If it's all good, the problem is the player.

Also be aware that certain instruments really don't sound "real" from a VSTi. It all depends on the sample...I hope you're using samples and not patches, then of course the articulations and layers in the samples.

Lots of variables to factor.