I don't use Ubuntu, but I seem to recall that there are many reports here in the forums regarding Ubuntu 10.4 and problems it had running SS that may be OS related. There should also be information here in the forums regarding where the log file is stored on the drive, as opposed to just viewing the recent entries from within SS.
If you're thinking of upgrading SS, i'd suggest going right to the 4.7 beta 1 instead of 4.6. You can download it from here:
http://sourceforge.net/projects/subsoni ... /subsonic/4.7 b1 is much faster and I think, it is also more stable than 4.6 It has one or two cosmetic bugs, but nothing that's a deal breaker. Send me a pm if you'd like to see it live and I'll give you guest access to my 4.7 B1 install.
EDIT: Got caught in cross-posting. The java heap issue has been fixed in 4.7 as many functions are no longer in the memory heap but are now handled by the database.