[LAU] UA25 and xruns

Greetings, I received my UA25 yesterday. The system recognizes it immediately, and audio I/O appears to be working fine. The MIDI ports show up in QJackCtl's ALSA connections panel, but I haven't tested them. Nice unit, but there's one big trouble with it: I get a lot of xruns from JACK. I've Googled for solutions, tried every suggested setting, got no joy. The only setting that works without xruns at all yields a latency of more than 30 ms (-p 512 -n 4 -r 44100). If I lower the period size to 256 I can get the latency down to 23.2 ms and suffer only an occasional xrun. Btw, the rtirq script doesn't seem to help this unit. I thought this device would get lower latency than the on-board soundchip, but the HDA chipset runs at ~17 ms latency with no xruns at all. Am I missing something ? The UA25's Advanced switch is on, the sample rate is set correctly. Any further suggestions ? Best, dp