Though I'm still very disappointed by the poor performance of TBS8921 I've purchased a TBS6925. This card has an excellent performance though only by means of the outstanding 3rd party tool CrazyScan. A big up to @crazycat!
Again, tools provided by TBS are of poor quality and buggy. According to the description "How to watch free MTV HD and MTV Music HD channels with 6925
" given by @Steven I tried to set the mux_id of the MIS on 12,5W, 12718H . But it didn't work. Only with CrazyScan I could tune to the transponder and set the id. The corresponding LNB for my steerable dish is on port4 (lnb4 or B/B) of a multiswitch. After examining the diseqc commands I discovered that port switching of TBSRecorder is faulty.
TBSRecorder for lnb4:
EO 10 38 FC
EO 10 38 FE
EO 10 38 FC
EO 10 38 FE
it should be:
EO 10 38 FC
EO 10 38 FE
EO 10 38 FD
EO 10 38 FF
same for the other lnbs, though there I don't need the tool