What is the order of rel subs?

Yes, there is a pecking order. The "ST" series is no longer available in the U.S. new except for the highest end models (Studio at $9,000 and Stentor at $5,000). The Britannia series is next (B1 at $3,000, B2 at $2500, and B3 at $2,000 with some variation for finish). Check out the Sumiko website referenced above for more details. I am most familiar with the "B" series as I own the B1 (12" driver and 500 watt amp) as compared with the B2 with a 10" driver and a 300 watt amp and the B3 with a 10" driver and a 200 watt amp). However, size of driver is not everything as the Stentor is only a single 10" but has a huge enclosure and the Studio is dual 10" drivers with a huge enclosure as well. The B and R series are the newest in the line with the R series made in China and the B series made in England. The older "ST" series subs were all downward firing and downward ported whereas the new B series is forward firing but downward ported.

And the R series is forward-firing and not ported at all.

Here is the order from the rel website:
Stampede
strata III
strata 5
storm 5
stadium III
stentor III
studio III

That is certainly the order BUT does not include the new B series nor the fact that all of the above except the Stentor and Studio are discontinued in the States. There is some fudge factor, but it is probably about right that the new B3 goes in between the Storm 5 and the Stadium III, the B2 is probably close either above or below the performance of the Stadium III and the B1 is between the Stadium III and Stentor III. Certainly when I spoke with Sumiko about the models, they felt that the Britannia line represented a step forward from the "St" series and indicated that the B1 would stomp all over the Stadium III. I have not compared the two directly, but it seems as though the B1 does not have any advantage over the Stadium in absolute extension (13 versus 12 hz) but certainly has more power with a 500 watt versus a 200 watt amp. Unless the price was great or the appearance important, I think the Britannia line represents a "sweet spot" in the Rel lineup. I believe the B3 was the Absolute Sound mag's subwoofer of the year for 2006 which is pretty high praise for its musicality.

I've owned a Stadium II, a Stadium III and a B1. I sold the B1 and went back to a Stadium III. I think REL, now owned by Sumiko, has gone the home theater route. That may be fine for some people, but I'm a two-channel purist.

As noted here and elsewhere, with the new lineup it's really hard to tell what's going on with the line. It's also interesting that REL's top two subs, the Stentor and Studio, still share the Stadium, Storm and Strata basic configuration. So if they truly believe the front-firing woofer is better, why do their top subwoofers not have that design?