The B1 has a 500 watt amp.
It sounds like a setup issue with its physical location in the room/ settings on the sub itself. I'd go through the setup from scratch, so it's dialed into that room and with those new speakers.
REL B1
I just put my REL B1 into a slightly different system and it sounds very lacking in low bass (which is unusual as it sounded great with the same amp, preamp before). Speakers changed from Vandersteen 2CE's to Klipsch Forte's. Preamp is still Rogue 66 Magnum and amp is still Moscode 600. I haven't messed around with adjusting the settings yet, but I'm thinking it could be a phase issue? Also, I used to have the B1 in a corner and now I have it centered. Lastly, I was running out of room for my Richard Gray 1200 power station (weighs 55 lbs) and put it on top of the B1. Any thoughts on where to start ?.