I love my RA, but I can see why they cancelled it.
Mesa's already got several Swiss Army knife amps, and the RA doesn't look like one from the front but it's actually complicated as f%&# The RA's big selling point was Multi-Soak, which I don't think they ever really got around to explaining how to use - there's a reason there's one for each channel, and that's the reason the amp's so complex. You can't dial in a sound and then just use Multi-Soak like a master volume. They're volume controls, sure, but more importantly they are tone controls. And I think a lot of players don't understand that you just can't get the "loud" sound without blowing some air through a good cab and some speakers. Hi on the RA with the soak up all the way sounds good through a little 1x12, and I have had many a pleasant evening of wailing with a doobie and just that setup, but it doesn't sound or record like a Loud Amp.
As well, the reverb and fx loop just didn't work as well as they should on a big-name amp in the 21st century, and there's a switch for 6L6 but I never got the amp to sound anything besides awful with them. Since Mesa's perfectly capable of making good reverbs and fx loops and multi-tube power sections, I am guessing that there's challenges from Multi-Soak that they can't fix without compromising something else. I personally would be perfectly happy not even having any of these options, but it would lessen the curb appeal for a lot of guys.
I really like the sounds I get out of mine but I'm not sure I'd pay full price for it (IIRC I was at $2300 Canadian all in, I bought the first one in Ontario or Quebec for sure and probably the first one in Canada) again. Helluva bargain at $1200, though.
The reverb circuit saturates a bit too much if set to high (does this with the Head built in 2012, have not ramped it up in the combo built a year later).
As for 6L6 tubes, the Mesa 6L6 (Selected Ruby 6L6GCMSTR) have a dark and low mid sound to them. They are perfect for the Rectifier amps but just lack the chime of RCA or Sylvania tubes. However the RA100 sounds really good with SED 6L6GC. The RA100 Combo I just bought had the Mesa 6L6 tubes and it sounded muddy when I first tried it out at the store, then realized what was in it. Since I have a few sets of EL34's, I guess the bonus was the quad of 6L6 that I can use in my other amps. I was not aware of the FX loop having issues. Seems to function great on both of the RA100 I have. (if you have issues, try changing the V3 (Chinese 12AT7 to a NOS 12AT7 (JAN/GE or RFT) as they cost the same as the Mesa tube).