I assume that this is a two channel audio system. Now that Bryston preamp has two XLR outputs, one variable and the other fixed. The same for the two RCA outs.
Now I assume you are using the speaker in the ported mode. Saying a speaker can be both ported and sealed is nonsense, as the enclosure volumes are very different. However the speaker has to have the optimal volume for ported application, and so you should use the speakers in ported mode. As far as I can tell from independent measurements, the frequency at which the speaker is 3 db. down is 50 Hz or close.
Now since the Bryson has no sub out, you will have to be creative. Now you only have one variable volume out, so you will need a splitter. I would use an XLR splitter,
like this one. So the input is connected to the variable out on the Bryston. Then you connect one output to your power amp and the other to the XLR input in the sub. I would start be setting the crossover on the sub to 80 Hz. Your speakers will be running full range, and this is fine as they are capable speakers. That splitter I linked will drive up to four subs, but I highly doubt you will need more than two subs. I would use two, and this is not an AV unit that has a sub out. Two subs will capture the bass from left and right channels. I would set the sub volume by ear. I do find that most people turn up their subs too high which is not natural. I would start off putting a sub right next to each speaker. You can experiment with sub volume and crossover frequency, but I think I have given you the best place to start.
You did no say how your power amp is connected to the Bryston, or what it is. If it is connected via XLR then that splitter will drive power amp to the mains, and your two subs. You will need one of those splitters for each channel.
I think this is the best way of going about it.