I'm running the Jantzen Silver caps in my ST120 and it sounds awsome. I've been toying with the idea of running some Mundorf higer grade Silver Oil Caps. If you go to http://www.partsconnexion.com you will see a bunch of cap options that are tempting. Fun to tinker with these things.. Make sure you de-charge the amp caps before going under the hood. Lots of capacitance on a 120 amp.
So I'm running 4 of the Jantzen Silver caps and I'm also running 2 Audio Cap Theta .10uf in the 2 spots on the signal path (top of the VTA board)... Check this link and I have photos of the VTA boardMy ST-120
Let us know what you do and how it sounds.