The volume that you hear depends upon a number of factors including the programme material that you are playing and the settings that you have chosen in Bartok.
We need to know a bit more about what you are doing to try to help.
Are you using Bartok directly into a power amp or does it feed a preamp?
Do you have Bartok with headphone amp and if so does your issue affect headphone listening?
What volume setting is displayed by Bartok?
What line voltage out have you chosen to use?
Are you using Mosaic for replay?
You mention your issue affects streaming Spotify . Do you use other sources e.g. local files, internet radio, Tidal, Qobuz or Deezer etc ? If so do they suffer from the issue?
The software history resource on the dCS website does not show an update for Bartok . Of course that resource may need refreshing if this is a recent change. Unfortunately as a Vivaldi user myself I cannot easily verify it. Nevertheless dCS product firmware updates usually have version numbers in the form v.1.0.x or v.2.0.x etc. so the one you give looks a bit odd. Either from Mosaic or from Bartok’s front panel display please advise the version numbers of the software/firmware installed.
After having seen your post, I found and installed yesterday on my Bartok the following:
App Version: 1.1.3 (119)
Front Panel Version: 1.11
Control Board Version: 1.11
Network Board Version: 1.1.3 (506)
It worked fine for me. No issues with volume.
It sounds better too actually.
Maybe that helps.
The versions you list are as I expected. So I have no idea what update 0.64.506.0 is.
I can only guess that when doing whatever it was that you were doing you inadvertently changed the output line voltage and note that you have now set it to 6V which you say resolves the problem.
I would just remark that as you seem to be using the Bartok directly into your power amps with no preamplifier I would check that the typical volume control setting you use on Bartok is reasonable. The input sensitivity of your Classe amps is ( I believe) 1.4V for full rated output. Using a 6V input from Bartok may mean that you have to set the volume quite low for a comfortable sound level. As Bartok has a digital volume control and as these work by reducing the available bit depth low settings may result in an unacceptable loss of resolution. I would therefore suggest that , if necessary, you choose a line voltage setting that allows you to use the volume control for a comfortable sound level when set between -20dBfs and 0.0dBfs.