Hi Tammi
I just tried the Selmer Signature a few days ago. First maybe I should mention that the throat Bb on my clarinet (not a Selmer Signature) is especially good compared with just about any clarinet I've played. Actually I almost never use the side Bb for anything but a few unusual trill I sometimes use. With that for comparison, I thought the throat Bb of the Signature is a little stuffy. Enough to notice a small problem. But, I didn't think it was especially bad, and I've tried some professional models, just as expensive, with far worse throat Bb.
As you know by now the register vent is a compromise, and for the Signature model they probably went for something that possibly creates this problem. It could be a lot of things like size (diameter), length, location, etc.
It's possible that to achieve whatever it is Selmer is trying to achieve with the Signature model, it is just impossible to have a better Bb. Of course it's not impossible that it's a small design flaw they missed. Some of the biggest companies missed details (or chose, IMO, not the best compromise) on some very expensive models.
And a lazy tech will tell you 'It's just the nature of the Beast'.
I don't really agree with that..... More specifically, unless you find someone with exactly the experience of dealing with something like this, maybe it's better they don't try to fix it, and accidently making it worse.
I had a very specific problem with a very respected model of bass clarinet (you can read about in the bass clarinet sub-forum). If I had to pay someone for everything I did, it would probably cost a fortune. I invested so much time but that's because it was my instrument and I wanted to solve the problem. I don't think it is fair to ask a repairer to invest that much for a customer's instrument.
Unless.... are you willing to pay for it? Especially if there might not even be a solution. In the case of my bass clarinet (former) problem, I don't think there is anyone else who checked it so thoroughly. It actually took getting in touch with two bass clarinet specialists who consult Buffet to find out just the beginning of what lead me to my experiments. This might be the same for the problem you have too.
So it's not necessay that there is a solution to your problem, but it also depends on how determined are you to finiding a solution and what you are willing to invest.
Nitai