Bookshelf finish question -- watco and stain...

I built a bookcase for my brother and initially finished it with Watco
(cherry color), but the color was not quite as dark as he was hoping
for. None of the other Watco finishes looked quite like what he
wanted, so I used a General Finishes oil based stain to get it to the
right color.

Now, the color is about perfect, but I'm not sure what I need to do
about a top coat. I had planned on just using several coats of Watco,
but now that I have used the stain, I'm not sure that is an option any
more.

Can I use Watco natural to build up a finish without affecting the
color? Or should I just put on a top-coat of something.

If I should use a top-coat, any suggestions? The only top coat I've
ever used is poly.

Thanks!
YJJim