Rivera has reissued their S120 head, which is a dual channel, stereo w/chorus beast....I have the M100 which is essentially the same, but not stereo and doesn't have the chorus...but the gain stages and cleans are essentially the same...I think they're great sounding amps...lots of natural sustain in the upper gain stages yet good note definition.
As good as the amp sounds with medium and high gain's on Channel 1 (British), Channel 2's "clean" is awesome...Paul Sr, having worked for Fender during the early 80's, really honed his knowledge and application of the vintage Fender circuitry. The cleans on the S120's and M100 / M60 rival that of the Blackface Fender's of the early days....
I have a Boss DS-1 that was modded by Steve Dudley (the Doobtone) which was his 7th one he made to kick the preamp in the ass if I have to or need more out of the lead channel...which isn't too often....I agree that tone is very subjective, but I can't say enough good things about Rivera's after owning one and I'm very pleased with mine...