REL B1

The sub is 20 yrs old. Things wear out. It may play but it may also be deteriorated to the point of sounding as it does mow. This is always the problem with older equipment The only way to know is test it. Then too, these subs only had about 125 watts. My Rel 3 Storm was never quite loud enough.This is an early Rel which were not powered as they are today

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.

Right, power has never been a problem with this sub, regardless of room size. 
I messed around with some of the adjustments last night and it sounds a lot better. Probably a phase issue. 
I’ll dial it in with a friend of mine. Just need to figure out what each of the adjustments control. I should be hooked into unbalanced high level, right?

thanks for the input and keep and please keep it coming 

The B1 has a 500 watt amp.

Wow! I was thinking it came out shortly after the era with the Storm 3 which only had 150 watts. Even their top model was not a whole lot higher. My mistake about the power. But it is still 18 yrs old. .

Just need to figure out what each of the adjustments control. I should be hooked into unbalanced high level, right?

Hookup is right The adjustments are volume & Crossover with a switch for phase. The crossover defines the frequency you want the sub to kick in and provide more bass. The biggest help is to learn what a bass guitar, standup, etc really sound like. I played drums all my life and so I sat next to the bass amp and I learned a lot. Unfortunately I'm not sure I can explain the process that well. I do it strictly by listening. The main thing is to blend the bass into/with the mains. Fortunately, the Rels blend easily and well. I'm not sure if I have helped you very much.

The manual is still found online so that should help.  I'd go through the setup again and that usually gets you close.

The phase setting and the placement in the room vs. the seating position go hand-in-hand.  It's probably the most important setting.

The biggest issues with the ST family of amplifiers, which the B-series used, is they used tiny little board-mounted components that are almost impossible to repair when they fail.