I don't own an R55, but I do own a TBR-2SL. Rivera tone is incredible, esp. if you like nice full bottom end (more than J-Lo after Thanksgiving dinner). I know the TBR's are very "cabinet specific". Some will sound good, and some will sound like sandpaper in a blender...even though they are both 4x12's with Celestions. It's kinda odd, but well worth the cab hunt. You're lucky- Rivera already built that cabinet. :-)
Rivera customer service is unmatched in my experiences with them. When the owner of the company takes the time to educate you on biasing your amp and takes your parts order at the same time, you know you've got a helluva company behind you. If you've got a question about a piece of their gear, they've got an answer; whether it's one of their regular techs or Paul Rivera Sr. himself.
There are a few Rivera owners/players here on the Forum. A few have also run thru my TBR (Simon gets the award for "Turned Up The Loudest On A Dare" from Jemfest 2002. LOL). I'm sure some other comments will come in. I'm pretty sure
Bob Oakman's wife is still a tad jealous of his feelings toward his Rivera. :-)