What is the order of rel subs?

The Rel web site has all of this in detail.
http://www.rel.net/index2.htm

Go to Sumiko web site for infomation on the Britannia Series and get a B3.

http://www.sumikoaudio.net/rel/index.htm

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.