Over the years we have tubed LOTS of the Rivera amps including the older "R" and "M" series amps up thru the Knuckleheads and "K Tre" amps. The build quality on the Rivera amps is a couple notches above average and we have seen VERY few Rivera amps that have had any issues related to build. They do have their own distinct sound with a good clean channel and depending on the model the drive channels go from mild to "Ninja Boost" wild. I wouldn't really call them organic sounding but they are warm and the drive circuitry is not too fuzzy and maintains good definition.
There are a few Rivera amps that use 6L6's for power but the majority of Rivera amps use EL34's for power and we use the JJ E34L's almost exclusively. For a number of years Rivera used the E34L's and we still see some new amps coming thru with them. The E34L's really make these amps dynamic and punchy. We will sometimes use the JJ KT77's if the mission is a chunkier low end but an EL34 type tone is still desired. The KT77's are a bit bigger in the low end but not quite as tight and aggressive as the E34L's are.
We do on occasion use the JJ 6L6GC's in Knuckleheads, Boneheads and the "M" and "R" series amps if the player is looking for a bigger low end and more of a Fender type clean. There is enough sweep in the bias to do this as long as the grade on the 6L6GC's is cooler. They do sound real nice this way, very fat and full and if you're into metal and are looking for less midrange this is a good option.
The small additions like the 25 watt Pubster and Clubster which use 6V6's for power are very nice and the JJ 6V6's really pump them up. It's nice to see Rivera and a few other companies using 6V6's again for low powered amps. Rivera is also making the Venus line using 6V6's and 6L6's and these are one of the only real class A amps on the market. So when you see one make sure to plug in and audition it. They are not for everybody but if purity in tone and big dynamics are what you crave give them a try. They are not wimpy by any stretch, the front end is quite similar to other Rivera amps and they have quite a bit of gain on tap.
All Rivera amps are adjustable and they are easy to adjust. You can use the generic bias video we did on the Marshall 800 for reference because the procedure is identical and all you need to do the job is a single bias probe and a multimeter. Generic Bias video using an 800 Marshall.