i'm not quite sure if this can be called a bug and it'll need further testing, but:
i get the impression that in some certain cases .51 handles sample/vst volume differently than older version do. experienced much lower sample-volumes than it should be while playing some older modules live last weekend.
If you ask me, certainly a bug if true - and a major one as well. To be investigated - I've also noticed something similar with older mods.
Edit: Results of investigations: For me, the likely cause for the difference was the value of pre-amp setting in sound card setup. The pre-amp value affects only samples, so playing a mod with different pre-amp value can make the VST/sample volume ratio to be wrong. To think of it, this is rather horrible behavior. The RC3 mixmode ignores the setting, however. Please report whether this is the cause in your case as well.
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Quick bug fix build(1.17.02.52) is available:
[Fix] Shouldn't need winhttp anymore.
[Fix] Fix to faulty plug file existence check.
[Fix] Sound card-options buffer length value validation.
[Fix] Yet another IT style-playback fix.
Executable hashes:
MD5 : 8f719b62f323c4d407e1703e590e239d
SHA-1: b16bf498f80b5f0833016f7571069b50a1201f53