Volume

[Axel-Erfurt]

With sf2 it is much quieter compared to other plugins. How can I increase the volume?

[AZSlow3]

Not that I know many GM sf2 plug-ins... In fact the only other I have ever used was TTS-1. And I have not noticed problems with low volume. That is SF2 specific, some are quieter then other. With published MIDI files and my preferred GM SF2(s) I am getting around -6...0dB.

May be you are comparing with plug-ins for single instrument. GM synths like FluidSynth are tuned to play multi-instrument files. Obviously single channel should not be maxed by default in this case.

You can control volume with standard MIDI CC7 (on particular channel). MIDI Tool in REAPER can send it more or less convenient way. Note that CCs are normally a part of MIDI data. DAWs can auto-lookup (and set) the last value based on time line, that depends on used approach (f.e. MIDI Tool send it on load or change/envelope only).

[Axel-Erfurt]

I use FluidSynthVST and SFZero in Linux / Reaper,

With the same sound font and the same midi volume, SFZero reaches -4.6 db and FluidSynthVST -16.4 db