i'm no expert on the 'jazz sound', but the way i'd approach it (my audio setup is completely stuffed at the moment, so i can't test it to see how it would sound) is this:
for jazz you don't really want any noticeable distortion, but for the vintage jazz sound, the amp would still compress a bit. the older amps also were not very good with high frequencies (or very low frequencies for that matter), so assuming the caps plugins don't do what you want (i find very few 'amp sim' plugins sound right if they aren't being used for rock or metal) then maybe use some lowpass and highpass filters to roll off the extremes (especially the top end) and then a parametric filter to boost around (i am guessing) 150-300hz to simulate the 'poor' cabinet resonances and so forth. if it were me i would still use a soft distortion plugin very slightly to give it a bit of life, but that depends on how clean you like your sound. you should also put a bit of reverb on, especially if you are using headphones. i don't know how flexible the routing is on jack-rack etc, but i would also try to roll off a bit of the high frequencies of the reverb as well to get rid of the 'sparkly' reverb sound (maybe the reverb plugins can handle this internally, i can't remember).
so what i would do is:
guitar input >
compressor (reasonably strong) >
high pass/low pass filters >
parametric filters >
distortion >
reverb >
low pass (if possible and necessary)
hope this helps
porl