Gfloyd,
Hmmm, I don't have any experience with the Monster Sigma Retro Gold so I couldn't say how any of the Acoustic Zen cables would alter the sound of your system.
However, the attributes you're searching for - "high end to open up a little bit and the midbass to tighten up" - are what the Silver Reference II excels at. Also, they create a very large soundfield.
My only fear is that inserting the Silver Ref II into an all solid state system might result in an overly lean and dry sound, especially with B&W 801 Matrix IIIs in the mix. But that's a very subjective thing. With a tube DAC and preamp I felt the Silver Ref II was a bit lean for me, but it seems your taste and mine may be very different so it could work for you.
I still have the Silver Ref II and have thought of selling it. I'd like to state this openly rather than approach you through private e-mail so it won't seem like I have a hidden agenda. Heck, I'm not sure I even want to sell it. The important thing is how the AZ Silver Ref II differs from the Monster Sigma Retro Gold, and I have no way of knowing if it will be a positive or negative change for you.
Maybe someone can comment on the Monster or give other suggestions. There are lot's of good cables out there and you probably would be able to achieve your goal without paying for a used Silver Ref II.
Something to consider is that the VK200 is on the dark side as well, but that's what I like about it.
Good luck!