ReplayGain is implemented for DSD tracks. It uses current DSD2PCM parameters even when playback mode is set to DSD. Output bit depth parameter in foobar converter plugin is for that, foo_input_sacd can't do this because it's not an output plugin.
For continuous conversion of single track sources set "File->Preferences->Advanced->Tools->Converrter->Thread count" to 1 and select tracks with the identical channel counts and samplerates.
Partially fixed in 2.0.20 when number of converter threads is set to 1. See readme.txt.
Thank you for discovering this. The part of code that prevents clicking was lost from plugin codebase. It works only for SACD-R ISO images. It it still clicks please share the source.
The other way is to setup any resampler in Preferences->Playback->DSP Manager panel. If you use Multistage DSD2PCM converter try to use DSD2PCM output samplerate as 1/8 of the DSD source sanplerate. For Direct DSD2PCM it's 1/64 or 1/32.
The latest versions of foo_input_sacd plugin use default foobar resampler for 44100x -> 48000x conversion. Not sure, but that might be the reason.
I moved it to https://github.com/manisiutkin/MPD
In 2.0.18 this volume adjust is back. Will not work while replaygain info is set.