REL B1

I would move the sub around.   Maybe back in the corner if possible.  Forte are very efficient,  some subs can't keep up with them in larger rooms.    It's probably a gain mismatch.  Are you running the sub speaker level or preamp level?

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.  

Yes, I’ve been referencing the manual, although still a bit confusing. Plus, I do get that it’s better to place the sub in the corner, which is the way I used to have it. So, like I said before, will mess around with moving the Fortes in a bit to squeeze the sub into the corner, start fine tuning the controls. I’ll let you know when I get it dialed in right. Thanks again !!

Have you ever tried the REL with its Speakon connector connected speaker level?   I have my REL set up this way and have set a few others up speaker level. It's REL' s recommended method of connecting their subsidiary.

Forte radiate front and rear, you could have some cancelation of bass freq going on there.    Put the sub in your listening position and crawl around the room.  The position where it sounds best is where the sub should go.   

In my experience, placing a sub in a corner can greatly increase boundary reinforcement, but at the cost of uneven frequency response.  I have had better luck with placing the sub along the front wall, inboard of one of the speakers at about the 1/3 distance line.