[LAU] problems using rosegarden, amsynth, and hydrogen

hello, I'm trying to write a track using rosegarden as the sequencer, hydrogen as a drum machine, and amsynth as a softsynth. I have written a tune in rosegarden, and connected its midi output to amsynth. The audio from amsynth then goes through tapiir using jack and then to the sound card. I also have hydrogen running at the same time, using the jack transport to sync the two. Everything works OK when I'm making tunes in the matrix editor and then playing them, but if I try to add a track by recording from vmpk into rosegarden, it produces a repetitive deep bass sound through amsynth. (I checked that if you turn down amsynth the sound disappears.) This only happens when hydrogen is playing as well - if I take hydrogen off jack transport, then the sound disappears. Any thoughts on what's going on here? Thanks, andy baxter.