Grishka - the last two voices V3, V4 in Nigel's abc should include some extra information to print correctly

In 2.1 octave=-2 as well as the clef=bass will treat the notes as being two octaves below the written notes (eg written D is treated as D,,)

In 2.0 (and 2.0draft) you'd have to add middle=d as well as clef=bass to get the same effect.

It looks as though the concertina.net converter assumes middle=d for the bass clef, though I don't think any of the standards make that default. The mandolintab.net converter applies the 2.1 standard - it needs either octave=-2 or middle=d to print the bass clefs correctly.

Mick