Thanks so much, ladies! I did end up going to get an adjustable chair today, because I figured I would need that anyway, regardless of whether I keep and modify my existing table or buy a new one. I was using an old dining chair that sat too low and had no lumbar support.
My existing table is 36x60, and it's solid wood. There is no pull out area. My machine is a Brother PQ1500SL with an extension table, so it's fairly large. It also has to sit flush with the front of the table in order for the knee bar to attach.
With the new chair, my sewing position is wonderful now, but I'm not yet ruling out a whole different work surface for my machine.
I am going to check out some of the links you all posted and explore the possibilities.