Man this is a tough question, but I`ve had three Shiva`s over the past few years, both EL34 and 6L6, and one combo, and I`m noticing a big similiarity between those and my K55 (late 90`s model). Anyone here AB`d these? Same fat low mids, etc, but the K55 has a gain control on the clean channel which to me seems like a plus. Only problem, no verb on the Rivera, but the effects loop has a send and return level which makes it compatibale with any unit sold. BTW, I haven`t had a Shiva in almost two years, so my tonal memory may be off with all the amps I`ve bought. I`m hoping I don`t jones for another Shiva after Hurricane Rita, LOL, funds are low.