To play sampled instruments try linuxsampler which can play gigasampler, SFZ, and SF2 sample formats. So if you have a base in one of these formats you'd be in business. Scarbee once sold gigasampler stuff, but unfortunately (from the linux point of view) that all got converted to Native Instruments format and it is no longer available as far as I know. You can use Native Instruments in Linux with wine, though I'm not sure everything would work.
Also, I'd recomment using a audio-oriented linux distro like AVLinux, kxstudio, or DreamStudio. These have lots of audio configuration tweaks and audio software pre-installed and will save you a lot of trouble compared to using a regular linux distribution like Ubuntu.