Rel has had the ability to use either line level or speaker level input for a long time. The exception is some early class Ds have to be hooked up differently.
Early Polk had a different setup too. I can't recall all the details but it had to do with the added wiring in their circuits between the two speakers.
A LOT of the early RELs had very limited options on their plate amps too. Some people love the things. Many of the people I know had an issue with the open
bottom passive and no way to decouple the sub without adding a slot after the fact. Our little club had a fix for VMPS and Rel subs that was about 60.00 usd
for a pair of slots and then whatever for the decoupling and or vibration control for tube/valve amps and playing records. They didn't have rumble filters for
using records either.
It's a good idea to add vibration control to the tube pre and power amps you use. It can add a LOT of longevity to some very expensive temperamental
valves/tubes and it never hurts to stop a stylus from bouncing across your new record either.
Personally, I've always split the line level out of the preamp BUT I've always had the option of XLR or RCAs too. For DBA or multi-subs I always used a second
control preamp like a DCX 2496 (6 XLR. 1/4" or 1/2" outputs plus the ability to daisy chain) or a Flex, the DCX has adjustable output to the plate input also.
I can say this about Rel subs, they have nice paint, look impressive stacked, and can be VERY expensive. Not my cup of tea, but then I'm a coffee drinker for the
most part.
Have fun and my best Regards