Well, answering to myself - but in a positive way at least, so some others with same problems might benefit from it.
After several glitches with Ubuntu-Studio I resorted back to good old mainstream Ubuntu 12.04 32-bit as principal version for my Lenovo IdeaPad S12.
After successful installation of the main system I additionally installed the low-latency kernel version and jackd2 (as the newer version).
Mind that the successfully installed low-latency kernel version typically goes to the "older/other versions" grub entry, so you will need to stop grub simply booting stereotypically in order to use it. (left shift key right from boot start)
Now the adaptations that Ubuntu-Studio presumably had made by itself were necessary, derived from the various web pages on the net. I report the changes that worked for me. Any followers might do this at their own risk!
# add root and me-myself as members to the "audio" group
# add to /etc/security/limits.conf three audio lines:
# in /etc/pam.d create the file "audio-session" with following content:
@audio - rtprio 95
@audio - nice -19
@audio - memlock unlimited
# in /etc/pam.d create a file according to your used window manager (for me: "xfce") with following content
session required pam_limits.so noaudit
# setup of jackctrl options:
session include audio-session
- deselect D-Bus-Interface
- turn on real time processing
- switch on 16-bit-mode
- choose appropriate hardware for ALSA, incl. the digits after the colon: for my Sb X-Fi => hw 1,1
- check in the message window if jackd really is invoked and keeps on running
- ATTENTION: after a standby/wakup the system erroneously changes the soundcard numbers! Therefore check with "aplay -l" which ALSA numbers are valid; reset them accordingly in the ALSA hardware choice, otherwise you will get trouble by re-invokation of jackd by JackCtrl.
# setup of QSynth:
- audio interface: jack
- set correct rates
- at least for starters: use very few effects
All in all, the rather weak IdeaPad works very stable at latency values of less than 10 msec, with only very occasional XRUNs that don't seem to matter too much. This is true both under XFCE and Gnome Classic (no effects)