i'd go with either 110 or 120 adjustable stem. you can pick up a "zoom" on ebay
for around $15.

I would suggest tilting your Brooks saddle up slightly more and try that. Sometimes, if the saddle is flat, you tend to slip forward, and may compensate by tensing your back muscles. If you look at the Brooks saddles of many people who have had them for a while, you will see that there is a definite upward tilt of the nose.

Be aware that a longer stem that encourages you to lean further forward may also compromise the position of the Brooks saddle. If you already have it tilted up, it will likely put extra pressure on the perineum area. If it remains flat, the stem may solve the issue, but then you might spend money on a stem for no good reason.

I have really tight hamstrings and a flat saddle will leave me with an aching back. I need my saddles to curve up in the back.

That said, it is a good idea to tip the nose of a Brooks up. Eventually it will develop that comfortable curvy shape.

But meanwhile, it might be worth taking some yoga classes to work on your hamstrings ... if they're tight.