The amp is not a Fender blackface by any means. The mark series amps on the other hand are somewhat derived from Fender circuits where as the Royal Atlantic, Electra dyne and perhaps the Stiletto were sort of modeled after the Mars lander from Brittan (I was going to type the word Marshall but some are offended by such language).
I may have written past my limit here. Sorry for long post. Note: I got my RA100 head working again. Must have been some dust causing an impedance issue on one of the relay terminals. Cleaned the amp with compressed air and seems to be in working order again.
It is obvious the cleans will not be clear an pristine as the Mark V or the Roadster on that regard but after comparison, the clean channel of the RA100 is very similar to the Roadster CH set to fat, channel power at 100W and tube rectifier tracking. The RA100 is in your face type of amp and will respond with pick attack as it is more forward in character. The clean is very versatile. I would call the character smooth and bold as it does seem to emphasis the low end similar to the Roadster. It is not dark like Roadster but similar. Hard to say how much sag is there, you can hear it when you decide to blow out your ear drums, more so present on the Hi/lo channel than on the clean as it has huge headroom. It is something you need to get used too with the RA100 and once you do the rest seem like they are missing something. The Roadster in comparison has a bit more sag to it especially with the spongy power setting which is similar to the Mark V variac mode. The RA100 does not have a variac control or extra tap on the input transformer. The output transformer of the RA100 is also different and much larger than that of the Roadster or Mark V. When you consider the RA100, think vintage tone except for the Hi voice of the second channel which is more of a modern push on the vintage character. The Mark V ch1 tweed setting is far more forward than the RA100 Clean channel. For blues style as well as vintage rock I prefer to use either of the RA100's which is the reason why I bought the first one. However, since I have loaded my RA100 combo up with Mullard CV4004 and Mullard EL34 (V3 is the only odd ball which is a 12AT7 for the effects loop buffer) the amp sounds incredible like a hyper Marshal JC something or another but only better as it still has a Mesa character to it. The Mullard EL34 are definitely hot tubes, Tung Sol EL34b are a bit milder and almost close to the SED EL34 (probably the best I have used in the RA100). I found the Mesa EL34 to be a little harsh sounding, not exactly brittle or sterile like you get with the Mark V, but probably a reason why some may not like the RA100. The amp sound way better with Mullard (re-issue), SED, or if you so desire to pay the extra coil get the Mesa EL34 STR 450 Siemens NOS. I would rate the Mullards second to the SED (which are now NOS and out of production). Mullards on the other hand have not done the typical EL34 tube rattle yet. It will put a little hair into the clean channel though. As for other preamp tubes, the Mullard reissue 12AX7A long plate is nice but some can be noisy and make a ping noise with channel changes. At least there are no Cathode follower circuits used by any of the 12AX7A tubes (V1, V2, V4, V5, V6 and V7) so you can use just about any 12AX7 tube. Tubes can be expensive, I would probably start with the Mesa EL34 tubes and move on from there. More than likely the amp will have stock tubes in it if it was used as a rental. Stock Mesa tubes will be marked as STR 447. One tube to avoid would be the Mesa 6L6 STR 440, I thought those made the amp sound like poo. A better alternative that rings out as a great 6L6 that I have used in the head was the SED 6L6GC, the amp still sounded like it had EL34 but cleaner so the basic tone is not lost. The Mesa 6L6 tube has a different characteristic that seems better for the Rectifier amp or the Mark V.
As for the Mark V, I did not bond with that amp until I figured out how to reduce the bias on the center pair of power tubes so I can keep from red plating Mesa branded tubes, I did that after I converted the head to combo, then installed the Celestion 90W Crème Alnico Speaker. That is sweet combination. it is still a bright amp but now it seems to be less brittle. The speaker helps a great deal though.